“Gooder boy” can be perfectly acceptable if supposed to be jokey and deliberately wrong English on purpose, as a play on the phrase “good boy” for meme reasons or just for language humour.
While the grammatically correct words are ofc “better boy” it wouldn’t make sense to have that in the description if the point of it is to be a riff on the pet theme because people would say that the dog is a “good boy” but you wouldn’t say of a dog “he is the best boy” or “I never saw a better boy” that’s just not the way people talk at least not that I ever heard.