Speaking as an avid metalhead, the game is funny. As far as I'm concerned, it's a great action-adventure game, that is very funny, and it has quite possibly the greatest soundtrack I have ever heard.

But again I am a metalhead so of course I would say that.
As Secret Fawful has already said, this game is merely aimed at a different demographic. I don't think that games like Brutal Legend are going to become all that common. So, I don't think you need to worry about Tim losing his "magical touch" because he makes one bad game.
Telltale Games have made a whole slew of mediocre games - such as their poker game, their CSI games, etc. Yet I do not hear anyone claiming that Telltale Games has "lost it". You don't need to be so melodramatic. I'm sure Tim will make a fantastic game later on that you will enjoy (and believe me, I enjoy his other work just as much as you do!)