Maybe that is why he asked Alphys to make a body for him, to not to be so weak when he encountered humans? Because he wasn't born a robot, that much is obvious. Apparently he transferred his soul onto that body.

At least that is what I deduced. He said I was in the right track but it seems I'm missing something about his true nature.

[A stellar moment in his stay in the Vault was to accurately deduce Mettaton was not always a robot. Not that it was too hard, tinman wasn't exactly being cagey about that]