I know he's a good actor so i'm not blaming him, just what he was told to do.
The character seemed to be Renfield from "Dracula dead and loving it".
The constant tongue flicking like a snake, the "haha i'm evil, i love evil!" attitude, the evil faces he'd make. Just so silly.
I agree, his character in the book is far more interesting and tragic as the reader and the characters consider the possibility that he was wrongfully imprisoned and think that he died in prison until the dramatic reveal when it’s revealed he’s pulling the strings, the film ruins the twist by having Barty Jr in the house with Voldemort and Wormtail at the beginning and also his attitude in the court scene, in the books he was screaming he was innocent and pleading with his father not to send him to Azkaban.
The film ruined his character.