Full disclosure: this kind of panel just doesn't have any appeal for me whatsoever. I Know a lot of other people like them, but I'm sorry, I just don't see the fun in sitting a room watching some random kids cosplaying as characters from a show. I can just walk through the halls and see that, right?
Still, I've caught the end of several such panels while waiting for ones I was in to start, and this is pretty much what they always felt like to me. People who aren't good at improv trying to work off suggestions from people who ALSO aren't good at improv. I'm sure there are exceptions, but not among the ones I've seen. And make no mistake, I get that improvisation isn't easy. There's a reason all the panels I take part in are either mostly showing videos or reading of a carefully prepared outline, after all. But that doesn't make these other panels any less of a chore to sit through.