Chewbacca Jones said:
Mutt doesn't bother me, but he's not lead-character material.
I didn't like KOTCS, but it had little to do with the Mutt character. I thought he was dull, but not terrible.
But not matter what you think of KOTCS, Mutt just isn't a remotely compelling enough character to hang a franchise on. He's only interesting tangentially, due to his relationship with Indy. What does he possibly bring to the table as a solo character?
Indiana Jones is an extremely unique and entertaining action movie character. If nothing else, he has specialized abilities, a specific skill set and esoteric knowledge that enable him to participate in a weird and imaginative universe.
Mutt, far from being "an expert on the occult" and "obtainer of rare antiquities" is...just a motorcycle enthusiast. There is nothing there to hang a movie on at all. What could he possible do that would compare with Indy? What could he possibly bring to the table?
Mutt knows nothing, can do very little...where's the movie? You'd have to take him outside of the Indy universe for him to be able to hold his own. And then, what would be the point?
You might as well make a movie about James Bond's dentist.