Both ranked and player matches have similar but different uses as far as leveling up goes. Player matches teach you how to adapt, since you have more time to do so and the adaptations you need to make become more apparent. Then ranked teaches you how to adapt quickly, because you have to adapt quickly to do well in that mode. Plus in ranked, there's something on the line. Yeah, it's not like it matters much, and it's not exactly like putting the lunch money you saved up to play at the arcades on the line like I did when I was a kid, but it's something that can make you more mentally driven to make those quick adaptations and not be lazy.
But I'm speaking mostly from my experience with AE here. Ranked in Marvel probably isn't as good at helping you learn how to adapt quickly, since one off games in Marvel seem random as fuck regardless of how quickly you can adapt. I figure it does force you to learn ho to play very solid in general to do well though.
Anyway, I'm doing better thanks to the advice some of you have given me. Starting to get used to a lot of things, but I think the added elements of controlling space in the air as well as how drastically assists can change each characters options are things that I just have 0 experience with in fighting games and are things I'm gonna need a lot of time to get used to.