One way in which you might participate is to explore what the other person finds so fascinating about the topic. You may think its idiotic but they clearly don't, so rather than try to convince them of your point of view, find out theirs.
Typical start "Oh did you see who got voted off idol last night!?"
Response, "I haven't been keeping up, but it sounds like you were surprised by that, did you think that was the wrong choice?"
Now you have done two things, one pointed out you have nothing to add in the discussion and two not made it seem like they are an idiot for watching it.
They may have a reason behind their surprise, find out what it is, find out what kinds of music they like and why, did this person sing songs from that genre? Did they do poorly?
As long as you realize it isn't a reflection on you that someone wants to talk about something you don't seek out, you can probably find some aspects of it that are at least entertaining.