"You look uglier with pants."

And remember to use the formal form (rather than tutoyer), because after you launch this there won't be much of a relationship left.

This sentence alone sounds a little odd in English. It is possible that it would make sense, but it would make much more sense if it was a comparative sentence in as 'You look uglier with pants than you usually do' or 'You look uglier with pants than she does'