The events of the episode actually make a lot of sense once you find out it was All Just a Dream Stumpy had. Kaeloo gets along very well with her alternate universe self, who technically has the same personality as her. Stumpy's subconscious has registered the fact that Kaeloo is a narcissist who adores herself. Mr. Cat gets along with Quack Quack's alternate self, who has the same personality as himself, and calls him a "real friend". Since Mr. Cat always gets annoyed by his friends, Stumpy must have thought that he would prefer his own company to other people's company and would love interacting with someone just like him. Stumpy, on the other hand, does not like his alternate universe self; as Kaeloo said at the end of the episode, Stumpy has pathetically low self-esteem.
More of a Fridge Tear Jerker: A possible reason for Stumpy acting more and more like Mr. Cat as the show goes on. Mr. Cat says that the psychopathic Mr. Duck, a Mirror Universe version of Quack Quack who is just like Mr. Cat, is a "true" friend. Since the episode was All Just a Dream that Stumpy had, we can assume that Stumpy thinks that Mr. Cat only considers people like himself as friends. Stumpy absolutely idolizes Mr. Cat, so it's possible he's acting like him on purpose to get Mr. Cat to see him as a "true" friend like Mr. Duck.
The episode turns out to be a dream that Stumpy had. In other words, Stumpy has dreams where he is disrespected, ignored, left alone, bullied for his hobbies, called a "retard", and told that Mr. Cat - and possibly the others - don't see him as a friend.
