You ever watch any sports where players are actively shit talking each other? Spoilers: it happens very often
Yup, doesn't make it right tho, especially after winning against the person. Again, I have yet to see a sports match where a player from a victory team immediately after winning starts sh*tting on the other team instead of the usual handshake sh*t. And certainly have never heard of it being considered acceptable.
sh*ttalking to rile your opponents, sure. sh*ttalking to legit hate on them after they lost to you is low