The only times I ever laughed during Mad TV, was during some of the Stewart sketches, and when Corky and the Juice Pigs were on.
And sometimes when Alex Borstein and Will Sasso were at red carpet events.
I think part of the problem with that show is they get a lot of people who CAN be funny, but don't have the ability to pull of impersonations, which is what Mad TV seems to focus on (aside from recurring characters like Miss Swan or Stewart).