Just me personally, I send a message if I played someone more than a couple of times. If they totally destroyed me but weren’t a jerk about it (i.e. rolling me into stun and then taunting), I usually send a “thank you for the matches” just to let them know there weren’t any hard feelings about losing and I’m still trying to improve. I usually receive friendly words of encouragement or tips when I get a response.
If the matches were close, I just send a “gg.”
If I just played a single match with someone, there really isn’t a reason to send them a message unless I want a rematch. It takes forever to type and send (even with a keyboard) and forever to load and read. It’s not uncommon for people to play through 50 different people in a single session, and when games are only a couple of minutes long, it’s just more work than it’s worth.