Firstly, people who say stuff like that are obviously durab and unknowledgeable. I'm not one to go about saying "YOU COULD NEVER WRITE SONGS AS DEEP AS TOOL." I agree with you on that; I don't like people who talk about Tool like they are untouchable GoRAB or some shit.
Secondly, since when was Maynard a hilarious black comedian? I know he was close to Bill Hicks and often incorporated black comedy into many of his viewpoints on the world but he was never a comedian. He studied art and wanted a career in art before participating in Tool; as far as I know he was always interested in profound and alternative ways of looking at the world. In fact, in most interviews/ press conferences/ etc I've seen with him he has always come across as a very, very serious person.
Plus anyway, comedy and specifically black comedy isn't lighthearted at all... it is often a very serious method of critiquing the world.
Thirdly, I agree with you in a way that Tool haven't done much for 'metal' and this is because I don't really place Tool in metal bucket at all. I see them more as art rock/progressive.
Fourthly, I definitely don't believe Tool are monotonous. They are anything but. Both emotionally and technically their music is very complex and layered. Can you give me any examples of where they use boring up-down-up-down three chord riRAB? Because as far as I'm aware their technicality is way more advanced than that.