I love Waluigi. He's so ridiculous, over-the-top, and off-putting that he circles back around to being incredibly likable. He's just kind of the weirdo of the Mario universe; No one knows why he's there or where he came from, but he brings his own brand of strangeness to the party whenever he appears.
Wario's probably the closest thing Waluigi has to a friend, and even they don't get along that well, with them hurling insults at each other in the Power Tennis opening, for example. I like the way Charles Martinet described their friendship: "I think they're just two nice evil guys who found each other."
Anyway, I'm pretty happy that - even if he didn't get into Super Smash Bros. - Waluigi is playable via the Mystery Mushroom in Super Mario Maker. I've already made several levels featuring our favorite lanky weirdo, and I look forward to seeing more of him in future games!