It's more complicated than that. I think this explanation is a bit of an exaggeration. I don't believe it's true by any stretch of the imagination that Marvel doesn't "care" about their villains. If they didn't care, they wouldn't consistently go out of their way to find top talented actors to portray them. Also, they wouldn't work so hard on the villains' costume designs, which are almost always excellent.
No, the problem comes from the limited screentime the villains tend to get. Marvel has obviously made a conscious decision that they want to develop their protagonists first and foremost, and as a result, the filmmakers end up investing the lion's share of time, energy and resources into giving the heroes top-notch portrayals. In almost every MCU film, there's no doubt that the heroes are the undisputed stars of their movies. The drawback, of course, is that there's a limited amount of time and resources left to develop the villain characters.
So it's less a matter of "not caring" and more a matter of priorities. Marvel obviously has made the protagonists their first priority. Consequently, only so much time remains for the villains. In this movie, Darren Cross has some very good scenes, but his screentime is fairly limited, so we only get to learn the bare bones about his character. The film obviously wants us to learn about Scott, Hank, and Hope first and foremost.
This pattern shows up in most MCU movies. To be clear, I'm definitely not someone who thinks that all non-Loki MCU villains suck. I think that's a gross exaggeration and distortion. For the most part, I think most MCU villains have interesting, enjoyable things about them (Malekith was the one I felt got neglected the most). It's just that there's not often enough time to develop them to their full potential, given Marvel's chosen priorities. In the future, if they want their villains to have deeper characterization, they should either give the villains more screentime or give them more character-driven scenes.