I consider it rude. Here I am licking my wounds, maybe cursing myself for playing so badly, and then here's the popup interrupting those thoughts, essentially saying, "Ha ha, let me beat you again."
Request Rematch After Winning

bro just says How did you defend that and come out ahead?! Can I get a rematch? that's so sus and like and subscribe pls!!!!!!!!!

To offer a rematch is giving the opponent the opportunity to play a new game and this is, strictly speaking, the prerogative of the winner. It is implicit that the loser would want to play again.