The story is confused and ultimately doesn't establish whether tumor made Brian stupid or not. Failing the SAT and getting down about it, and then hanging out with Peter and his enjoyable idiotic activities, made him look stupid but don't fit plausible together. The operation wouldn't have made him normal again, he could have to chose to go back to his old lifestyle if he wanted to (this makes his bitter, sarcastic comments about how Stewie ruined his fun new life by tricking him into having the tumor removed inconsistent with how he acts the rest of the time).