Sympathy for English speakers here please. "Pants" in English means something completely different from "pants" in American. Surely "pantaloni" really means "trousers", doesn't it? Pants = underwear in English English.
I can't imagine a situation where I can use this phrase... But maybe the funnier the better
The English should be: "You seem/look/appear uglier IN pants." "With pants" is not English (unless what is mean is that the person is holding a pair of pants, not wearing them, and that makes him or her look uglier).
Please give the option of 'with trousers' the mind boggles of people in ugly underwear here in the UK.
Trousers should be accepted, in british english we use pants to mean something else.
You look uglier with trousers should be accepted. Queen's English came before American English!