Glenn smiled and clapped? That's very cool! He certainly isn't a musical snob if he did that. In his position in rock/pop music he could easily look down to KISS and sneer. Maybe he's - unlike some celebrities - clever enough to realise that the makeup and show thing is just meant to be fun and it doesn't make the music bad.
Incidentally, when the original KISS made a surprise appearance at Grammy Awards in 1996 (before they had played a note in public), you can see Don Felder clapping and smiling. At 0:54 mark in this clip:
https://www.youtube.com/watch?v=ekttWq_XDow
But in his book he kind of puts KISS down. But it's funny, he says that the Eagles didn't move enough on stage (or something like that) when he saw them before he joined. And when he had joined, he says the Eagles didn't need a show, unlike KISS.
Anyway, when I read his book I was thinking that maybe his smile in that clip is more sarcastic than sincere? Timothy's facial expression looks rather neutral.