Well, apparently it's okay to sing theme songs and whatnot, just not okay to actually watch the shows or anything...Even one kid in 5th grade who claimed to hate Muppet Babies quickly started singing the theme song to himself, lol.
Similarly, this really obnoxious kid I went to school with in Grade 6 would CONSTANTLY sing, "This is the Song that Never Ends" from Lamb Chop on the bus home every single day for the longest time... but then again, that particular kid was... well, I can't really say he wasn't right in the head... he just had like a SEVERE case of ADHD or something.