X-card for audience members, no. IMO, audience members that are bothered by content at a table they are watching should just move on.That's definitely a take I've not heard before regarding the X Card. I'm confused by your meaning:
In your example, is that person who plays the X Card not also a paying player? Who decides what's important to the plot?
Have you had an experience like this? I'm having a hard time imagining a player X Carding a topic so vital to the story being told at the table that it can't be pivoted or modified slightly to make everyone comfortable.
If I as GM can't ask the x-card playing player what the problem is, how am I supposed to modify things in an acceptable direction, in a noisy room, under time pressure, with a scenario that was play tested with this content, while other not bothered players who paid good money wait?