Yeah his material and/or performances went downhill in recent years. I personally think he peaked in the 80s and 90s. Luckily I saw him on the "You are all diseased" tour in the late 90s.
I think the problem was he started getting to serious about politics. Rather than being a comedian that makes fun of things, he was starting to become an activist on a soap box. I think that kinda ruined him. The old Carlin rocked!