Some of those boxes have already been rifled through, for those wires and spare parts at the bottom. Not so much the clothing, although if Mettaton finds a cloak that's not too threadbare he might claim it and dazzle it up a little. But new outfits aren't in short supply, in the castle. Any kind of newer-than-medieval technology is.

Anything that might help to fix him, if he's broken again.

Maybe you'll find him as he's prying the lid off another crate, with a furtive, hunted look on his face as if someone might force him to stop. He's being greedy, he knows, he's being selfish, those aren't all for him - but just enough to be safe. Just one more. One or two.

Or maybe you hear his cry of surprise as he reaches into a box, and whatever's inside it whirrs to life...