Disney's Cruella seemed to be drawing a lot of inspiration from Joker, but the movie itself reveals that this was little more than a marketing trick. With both movies being origin stories for iconic cinema villains, then it isn't too surprising there appeared to be some shared DNA. That was amplified when the marketing machine for Cruella started up, as the trailers very much made it look like Disney's own riff on Joker, with tonal similarities and a story that took someone typically evil and painted them in a more sympathetic light.
Cruella was never going to be exactly like Joker, not least because Disney would need to keep things far more family friendly than that movie did. However, the comparisons to it - and also to the DCEU's Harley Quinn - suggested a certain amount of what to expect from Cruella's plot, main character, and overall style. Given Joker was such a huge success, then it would be a logical blueprint to follow from a villain origin movie, but that's not what Cruella does.
While Cruella does explain how Estella (Emma Stone) turns into Cruella de Vil, the way it does so and the trajectory of that is very different from Arthur Fleck becoming the Joker. In terms of character arc, the chief difference is how much Joker commits to its villain being, well, exactly that. Whether or not Arthur is still the Joker in future, or if a copycat emerges, the ending leaves little doubt about his transformation in that sense, with the bigger questions being over how real the events were. Cruella, on the other hand, becomes an anti-hero; there's a different villain to defeat, and while she does some things that cross the line, she's still a very long way off being the dog-murdering character from 101 Dalmatians, to the point it's hard to understand how she gets there (if she ever does in this timeline).
Tonally and visually they're rather different as well. Joker was very much inspired by Martin Scorsese's Taxi Driver and The King of Comedy; it's a dark tale that includes a lot of violence, but also tackles its subjects mental health and the reality of what's going on around him, all of which comes to the backdrop of a seedy, neon-lit Gotham. Cruella, on the other hand, feels like a much lighter affair. While it does touch on the personality split between Estella and Cruella, and people do die, there's a lot of comedy and flamboyancy to the film. Its 1970s setting meshes together punk and glam rock, and its aesthetic as a whole is pretty fun, bright, and decidedly un-gritty.
There are a couple of similarities in terms of plot and themes - both movies, for instance, touch on issues around class, while each has a parental twist that sees the main character seek revenge on the wealthy businessperson they believe wronged them. But even then, the depictions and handling of them are very different. For all this, though, the trailers for Cruella very much played up its similarities to Joker, honing in more on its ostensible edge and lead character's inner darkness. Even Cruella's director was aware of the Joker comparisons, and again, give that movie's success, it's hardly surprising why the trailers played up those elements. It just that in the movie itself, they're rather lacking.