If you still have the message, you can still reply! Something along the lines of “GG, thank you for the Match!” always works.
Writing during a match, it’s easier to do on PC (the game features cross platform between Xbox One, Windows Store, and outside of Ranked, Steam). I’ve never seen a premature GG sent in Killer Instinct yet though.
Tactics like teabagging, especially when done mid match, are usually meant to troll/make you angry. When you’re angry, you’re likely to make more mistakes and blow the match, which is what they want. It’s a psychological tactic, so never let it get the better of you!
This is also the case with insults and such mid-match; the opponent might just be trying to irk you and mess you up psychologically.
For insults sent post match, block, report, and move on.