There are times in the show where MPB seems like he’s very emotionally mature, almost the voice of reason when talking to one of the characters. Then there are times where either the butt of the joke or just really good at playing stupid. From his word plays, to his scatter brained ideas with Todd, to the crew neck/v neck discussion with Bojack. I think it’s just hard to know how to take his character
There’s a great, thorough description of him here: https://bojackhorseman.fandom.com/wiki/Mr._Peanutbutter#:~:text=Peanutbutter%2C the star of Mr,main characters in BoJack Horseman.