In concept a realistic artstyle isn't a problem at all - look at Smash. Mario, Pikachu, Snake and Samus all look like they might as well have been made for the same game, usually with even more detail than the design they're based on. MvC3's comic book style was super pretty to look at, but changing it up for the sequel wouldn't have been a problem if it was pulled off well.
But I guess the art team, the budget or both were lacking in some regard and it didn't turn out as well as it needed to. All you really have to do is compare it to Injustice 2 and it's pretty easy to see there's a lot of room for improvement, people have been saying it looks like a mobile game for a reason. That said to be perfectly honest I'm still mostly glad they changed the art style just so it's easy to tell apart from 3, but it's hard not to imagine how much better it deserves to look.
But I guess the art team, the budget or both were lacking in some regard and it didn't turn out as well as it needed to. All you really have to do is compare it to Injustice 2 and it's pretty easy to see there's a lot of room for improvement, people have been saying it looks like a mobile game for a reason. That said to be perfectly honest I'm still mostly glad they changed the art style just so it's easy to tell apart from 3, but it's hard not to imagine how much better it deserves to look.