Although Superstore didn't last very long, the characters went through massive changes through the six seasons of the show. Some changed personally and career-wise in big ways, while others only went through slight changes during the show, for better or worse.
Now that Superstore is over, fans can rewatch from the beginning to the end to analyze each character and their growth on the show. Based on some Reddit threads, some characters improved greatly while others just seemed to get worse and worse.
Sandra
In the earlier seasons of the show, Sandra was barely noticeable. She was timid, hardly stood up for herself, and people normally talked over her. Despite her tendency to fade into the background, she delivered some funny one-liners that made her stand out to the audience.
For a while, Sandra told lies to get attention from her co-workers, like the time she went along with the lie that she was dating Jeff. As the seasons progressed, her life changed and she no longer needed to lie to get noticed. She was married to someone she truly loved and got along with, plus she learned to stand up for herself. In the final episode, she confidently told Dina that she'd be Assistant Manager of the fulfillment center, bringing her arc to a perfect close.
Garrett
Garrett started out as someone who didn't really care about much, especially his job. He always made it clear that he was there to do the bare minimum and nothing more. His sarcasm was a dominant feature of his character, as well as his insistence on keeping emotions out of his friendships and relationships.
With time, he started to open up. He became friendlier towards Jonah, letting him share his apartment when the tornado had destroyed his and going along with Sandra when she was setting him up to be friends with Jerry. His relationship with Dina also improved when they finally both stopped avoiding their emotions.
Dina
None of the other Cloud 9 employees were as dedicated to working and maintaining store policies as Dina. As a result, she was a little too stern and overbearing. She even opposed walkouts, strikes, and union movements even though they were done to improve the employees' lives.
But underneath all that, she had a soft spot, which she didn't let out much until further into the series. She became more empathetic and even tried to help Mateo at the end of season 4, became best friends (sort of) with Amy, and stopped evading and burying her feelings for Garrett.
Amy
Amy's story began with her in a marriage she wasn't completely happy with and working a job she needed, despite how much it frustrated her. Character-wise, she was a little uptight and judgemental and cared a little too much about people's opinions of her. But, that started to change the more she interacted with Jonah.
Apart from letting loose once in a while and allowing herself to have fun, Amy took on higher positions at work, even though she was initially reluctant to. This led to massive career growth as she got a corporate position in the final season. When Jonah seemed insistent on a permanent commitment, she didn't simply give in either, having learned her lesson from her first marriage. It showed real growth that she took her time to decide if it was really what she wanted.
Jonah
Jonah had one of the most complex story arcs in Superstore. He started out awkward, always saying the wrong thing, and then trying too hard to fix his mistakes, only to end up making things worse most of the time. His overcompensatory behavior spilled into his relationships as he always tried to move to the next level, even when he wasn't ready for it, rather than talk out the real issue and risk confrontation.
By the time the series ended, he'd improved in some things and gotten worse in others. When it became clear that he and Amy weren't on the same page about their relationship, he opted to break it off rather than just go along with it and follow her to California. Compared to former relationships where he went along with things even though he wasn't happy, this proactiveness showed growth. From a scene in the montage, he also got into politics, finally making the changes he always preached about throughout the series. Other than that, he still tried too hard to fix things at the store rather than let them go and kept causing a mess.
Cheyenne
Cheyenne's debut on the show was as a funny, young, impressionable, and pregnant girl with a boyfriend who could hardly be considered mature. Even though she was pregnant, she couldn't quite grasp how big of a responsibility that was until Amy got through to her about the reality of diaper expenses and motherhood.
Being a mother certainly changed her for the better and her dedication to her job increased. Over time she went from a floor worker to a supervisor and she did her job well. She was a lot more mature too, but viewers felt like she also got a little snarkier and less sweet mostly because of her friendship with Mateo.
Mateo
Mateo's mean streak was a prominent part of his character in the earlier seasons of the show. He often made fun of his coworkers, even when it was insensitive to do so. For example, the time he called Jonah 'Trailer Trash' when the tornado destroyed his apartment and he had to live in a trailer. He was very competitive too and openly criticized everyone.
The audience became sympathetic towards him, after his run-in with ICE and it made him a little less mean. He was still snarky occasionally but it balanced out with some moments of kindness and genuine friendliness towards the others.
Glenn
At the start of the show, Glenn was the store manager. The position stressed him out a lot but he stuck it out in spite of that. He had trouble making decisions, especially with regards to the employees but he was very kind to everyone and was sort of a father figure. When he became a floor worker, he seemed a lot happier and he avoided taking on bigger responsibilities even though he had the experience for them.
Character-wise, Glenn didn't change much. He was still sweet, funny, and kind, but he did get a little less indecisive and more proactive. At the end of the series, he reopened his father's store, Sturgis and Sons, which indicated how far he'd come from running away from bigger responsibility.
Marcus
In the beginning, Marcus wasn't very likable. Fans found it hard to look past his ignorance and the way he always wanted to be best buds with everyone when it was clear they didn't want the same. He also had the tendency to overreact to things because of his insecurities, like when he made a big issue of the Zephra CEO being a woman.
With time, he turned out to be a good friend and improved to some extent. He also had a humorous side although a lot of it was weird stuff, like his entrepreneurial idea to make cheese out of breast milk.
Carol
Carol was generally an okay person in the beginning. As a side character, she didn't have much screen time in the beginning, except for a few random appearances. She had a tendency of going after men as soon as she heard of them having relationship issues, like when she called Adam when everyone found out Amy's marriage was rocky. She also told lots of cruel lies to try and break Sandra and Jerry up.
Later on, she got worse, especially after the Sandra-Jerry fiasco when she accused Sandra of stealing her boyfriend. She continued to get more aggressive towards Sandra, bullying her and bossing her around at work, and even tried to drop her cat out of a window. Sandra eventually learned to stand up for herself and Carol mostly backed off and threw her focus into suing the store.