There are various degrees to the act of cumming on a woman's clothing. You see some pics with cum on pantyhose, or a guy cumming simply aiming to cum on the woman's clothing. It's going to be different for each person's fetish.
For me, it's not the act of simply cumming on a woman's clothes, but rather, the representation of the spontaneity of the oral act. A woman, who is still clothed, opts to orally please a man. In the finale', the man cums on the woman's face and due to the unpredictable nature of the act, cums on the woman's clothes, as well. The woman, is unconcerned about the mess created, because her primary concern was to give her man a blowjob.