You could always argue that he is a moron (even with that, it's not excuse to me either), but what blows my mind is that apparently Barb fully buys this too.
She is a cantankerous harpy, sure, but also supposed to be a somewhat neurotypical woman. Not only didnt she catch on how this was just a rerun of all the previous million of fake girlfriends, but she thinks that his brain damaged, gender confused swamp monster has any chance to date any non-mongo woman.
You could chalk it to blind mother's love or being senile, but she believed a lot of shit in the past, and no mother's love could hide than since he began to pretend to be a bag lady he demolished whatever infinitesimal chance he could have.