You can't make the comic relief the main character
Once again this movie shows that a comic relief character does not work as the lead of a movie. Part of the reason people don't like this movie is that Mater is too much to be a lead character. He worked fine in Cars because he was used in small doses. The same with Jack Sparrow in the Pirate sequels, he is a fine addition to the cast for them to play off of but he doesn't work as the main character. No one wants to see a movie with R2D2 and C3P0 or Jar Jar as the lead character.
I can't think of a single movie where the comic relief/sidekick character has ever been able to carry a movie. There is a reason they are the comic relief, 2 hours of Mater just makes this movie a chore to sit through despite how good the animation is.