Is it wrong that I'm planning to wear my girlfriend's mom's clothes while I'm house sitting for them?

Yes.

I don't know about wrong, but it's awful strange. Why would you want to wear a woman's clothes? Do you plan on washing them also?

Umm....Yeah, kinda nasty to wear someone else's clothes that you don't have permission to wear, want to wear women's clothes? Buy some! Js.

It's unusual but no crime

That's rather unorthodox, isn't it?

Yes, it's wrong. But hey, you have freedom. Do what you please.

NO,  I do not think you should.