Honestly, I think it's just about bare skin exposure. Samus and Bayo have their breast basically completely covered when visible, while Mai's are hanging out all over the place. And Shulk, well, Shulk's a guy so it's more funny than sexual.

Quite true, indeed. Some people are saying "but what about Krystal? Sh'e just as exposed as Mai and she's in the game". Well, fun fact: rating boards actually consider fur as "clothing" for censorship purposes, so, to them, Krystal is actually fully clothed. They do this to avoid loopholes with characters like Donkey Kong, Pikachu, Sonic, etc., who would otherwise be considered "naked", which is why Mai in particular does not pass the rating test, but others do.