Kryptonians in DC Comics - namely, Superman and Supergirl - are best known for having human parents that influence their upbringing. However, when this element of their origin is changed, the result becomes tragic. Even having Wonder Woman for a mother couldn't save Supergirl's story in Elseworld's Finest: Supergirl & Batgirl.
The creative team of Barbara Kesel, Matt Haley, Tom Simmons, Moose Baumann, and Bill Oakley gave Kara Zor-El an origin that changed her life's trajectory. In the 1998 Elseworlds story, Elseworld's Finest: Supergirl & Batgirl, a fascist Gotham presents a challenge for this dynamic duo. However, the two do not begin as partners and Supergirl is incredibly naïve, thanks to growing up with a super parent and family.
As usual, Supergirl arrives on Earth, though she is the sole survivor of the destruction of Krypton and the subsequent journey. Kal-El made it to Earth, however, he ended up in the clutches of Lex Luthor, becoming nothing more than a science experiment. He never grew into a hero. However, Lex Luthor's reach extended into Kara's life. After arriving on Earth, the young Kryptonian was adopted by Wonder Woman and eventually joined the Justice Society. She grew up living with heroes. This left her open to major betrayal.
In this world, Luthor positions himself as a beneficial figure to Kara and the other heroes. He is seen as a humanitarian developing clean energy. He donated land to the Justice Society and even oversaw the design of the team's headquarters. To Supergirl, he was a mentor who was an influential figure in her life - going as far to be a father figure. Since Lex and Wonder Woman were both perceived as ideal parents who upheld the highest morals, Supergirl had no reason to be suspicious of anyone. This made Luthor's betrayal of her even worse.
While teaming up with Batgirl to "save" Luthor from the Joker, Supergirl discovers his secret: he had been using a deceased baby Kal-El to make a solar battery from him. This stuns Kara, forcing her to realize that this great man in her life that she's seen as a paragon of progress and goodwill is not just a fraud, but a person who has been using her and manipulating her. Clark Kent and Kara's upbringing by human parents is a big part of developing their humanity and love of it. It fuels their view of the world and their desire to protect its inhabitants. Having human parents effectively humanizes them. With a superhuman mother like Wonder Woman, and the lies of Lex Luthor, Kara wouldn't have long-term experience with lying and betrayal. After all, Diana is the Goddess of Truth and would have no reason to lie to her daughter or be anything less than seemingly perfect.
Lex Luthor is most often depicted as a figure who seeks personal gains at the expense of others. It's what makes him at odds with Superman. Here, however, he becomes a much more sinister figure, taking advantage of Kara's perception of everyone in her life as well-meaning, heroic individuals. While most of them still are, particularly in Diana's case, it creates a vacuum of goodness that is highly deceptive within Kara's upbringing. To be the best and most real hero possible, Supergirl needs human parents and a more human upbringing. This is what allows her to not just understand human nature, but how to be the most honest version of herself, prepared to face the challenges in her universe. Wonder Woman might make a great mother someday, but the environment she raises a child in - and the people around them - requires extra attention.