O uso da preposição vai depender do verbo ao qual ela está ligada.
Se usamos "wear
" (no need of preposition): She is wearing a beautiful hat.
Se o verbo for "put
" (preposition on): I put on my clothes in the morning and take them off at night.
Se usarmos "to be
" falando do estilo, cor, de forma genérica, etc. (preposition in, some say that the first preposion you associate with clothing is "is" due to the fact we actually are in them, or them are on us
):The lady in red. That man in a blue suit.