"You could stand to accept a few more compliments even if it does inflate that big skull of yours," is all Mettaton says on that subject. He gets the idea that Sans didn't really...accept his apology. He can't be sure, but why else would the skeleton ignore it? Not even...comment on it?

Sigh.

"Anyway, it's good you took your time, I suppose; because you missed all the 'excitement.'"

Which he does air-quotes for, mop parts notwithstanding.