These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Pete: Is he a horrible person who deserves everything he gets, or a Designated MonkeyJerkass Woobie? Which side you fall on seems to rely heavily on how much of a Woobie you think PJ is and on how blameless you find Goofy.
Bizarro Episode: The episodes in which Goofy reads to Max the history of various ancestors. Aside from the framing device, there is nothing to tie these into the series continuity and they play more like Classic Disney Shorts than Goof Troop episodes, up to and including putting Pete in the role of a traditional villain in four of them. Peg and Pistol are also used in the stories but multiple times are given roles counter to their canonical characterization, Pistol's character is never related to Pete's, and PJ is not seen once in any of these episodes, story or framing device.
Max's cousin Debbie is extremely popular considering she was only in one episode, as is the one-shot principal Mrs. Pennypacker. Several fanartists will draw the two of them along with Peg.
And definitely Peg, for the reasons given above. Even though she's a supporting character with only a few limelight episodes, all the fanart of her would suggest she was the main character.
Jerkass Woobie: Pete sometimes fits this trope depending on the writer. Sometimes his karmic punishment can tread into Disproportionate Retribution, and occasionally he won't have done anything wrong in the episode itself, which can make him appear to be a Designated Monkey depending on what parts of the show you've seen. But most fans will agree that he deserves at least some of his punishment because he's, well, a Jerkass.
Love to Hate: Pete has a sizable fanbase of people who love watching him get his comeuppance. He is loved for being an antagonist with depth, realistic motivations, a number of relatable flaws, and a hilarious personality. He is hated for selfishly and dishonestly mistreating the two most sympathetic characters on the show on a habitual basis (and others on occasion).
The Woobie: What happens when you take Pete and give him a son who is his exact opposite? You get PJ, the harmless innocent who suffers regular abuse from him. Usually child abuse, especially of the behavior-affecting variety, is enough reason to count on its own, but that is far from PJ's only problem (see Woobie.Western Animation for more details). Look at comment sections for videos and fanwork focusing on him, take a drink every time a fan a) expresses sympathy for him, b) rages against his abuser(s), or c) gushes about him earning his happy ending, and you won't survive the day.