It's hard to say as someone who's only joined the game last year, particularly since there's been very few villains apping during that time. Mostly they've been of a low-key nature. I think the player base can be understanding - the last time Masa played Ganondorf, as I understand he didn't run into any hero problems until he called for it as his send-off for Ganondorf. I think I remember their being a 'villain list' at one point in the past, but that's no longer a thing as far as I know.

It probably depends on how you intend to play the villain - low-key and biding their time or ready to openly commit evil? If there's no castmates for the character, they may be able to slip in. Otherwise, getting in contact with in-game castmates can help to get a feeling of how the character would work in-game. I do know that recently a Pokemon Special villain, Petrel was apped (I think the user name is watchyourback?) He hasn't introed yet, but seeing how things go with him could wind up serving as an example, for better ot worse.