Ok so like
when you go around and talk to everyone, all his employees love mettaton. They all talk about how nice he is (esp if you kill him)
So that means mettaton is ONLY mean to one employee- Burgerpants. The contrast is so strong that it actually feels very out of character for Mettaton even though its canon
so heres my theory
Mettaton has no clue he’s upsetting BP
The nickname, the songs, he just assumes burgerpants is in on the joke. He’s poking fun at him, maybe a little harshly, but is doing it because he’s trying to joke around with him and maybe even to show that he’s over the burger incident?
Like, i think mettaton can be a selfish and have a hard time thinking about how others are feeling, but i don’t think he’s downright cruel or trying to torture BP. I think he doesn’t realize that bp doesn’t think its funny.