POSSIBLE SPOILERS BENEATH FOR ANYONE THINKING ABOUT PLAYING THROUGH THE MOTHER GAMES
Porky acts like Gary in the Pokémon games, but exaggerated a thousand times. Porky is the ultimate nuisance in Earthbound, flipping you over at every given opportunity. Even when he helps Giygas he decides to screw him over too by releasing him from the Devil's Machine, thus making Giygas lose all control of himself. It's just in his nature to screw everyone over, which is why many people hate him. Even worse is that he's responsible for the deaths of Lucas' mother and Claus in Mother 3, many people have begun hulking and crying through the game because of Lucas' struggles (I personally found it to fall flat, but that's another story,) and those are all because of Porky. He's hated because he's a character that's made to be hated by the player, much like Gary, except that Porky is literally evil, where as Gary is just a nuisance. Shigesato Itoi has also stated that his actions come from bad parenting, which is also seen in Earthbound, and when such a kid gains such power he becomes very dangerous.
His role in Mother 3 was actually well received in general. It turned Porky into kind of a tragic and weak character, despite being on top of the entire world. When he had gained control he used the whole world as his playground, turning animals into robots, creating his own twisted society and army, it was just a big playground for him. Which is because he still had his child mentality after his time travels, heck, he literally has his own playroom. By the end of the game he's sealed inside a Absolutely Safe Capsule for all eternity, he has become immortal so there's no way he'd die or starve in there, and there's no way to get out of the capsule since there's no exit and no way it can be destroyed. Thus, he gets trapped for all eternity. It's so ironic, even if he's been the biggest ******* throughout the series it's hard not to feel sympathy for him.
In my opinion, Porky is one of the most well rounded and best pulled off villains in any video game ever. Like Shigesato Itoi himself says, Porky is like a poem, there's so many sides to him and so much thought put into him.