That pretty much tells Mettaton what he needs to know. Like he'd said, being human was hard work, and Napstablook could hardly be expected to know anything about it.

"That means that you are hungry, darling. I am assuming you didn't eat the lunch you were packed, then?"

Even if he knows that Blooky is new to the whole thing, Mettaton still fusses over his cousin, because that's just the way he is. It's probably familiar behavior, him asking questions, explaining things with a mixture of patience and exasperation. One thing is certain--as always, Mettaton means well.