Depends on how you define a 'hero'. One of the villains from All Might's world, clearly has specific definitions of "hero" and "villain".

[Trunks doesn't usually think of himself as a hero either, even though it fits. It's more he just views it as doing what he has to, and doing what's right. It's part of who he is.]

But as mentioned, it is a Guardian's--and the Deputies-- job to keep peace. Someone comes here and was a villain in their world, or a bad guy? Long as they don't break the laws here and don't make trouble, they get protected just the same. A 'hero' can't go after them just because back home they were the bad guy.