Personally, I'm a big Krystal fan. But not because of the whole hawtfurry thing. I just genuinly like her. I played as her in SFA when I would play Co-Op, so, I like her. And YES, Krystal has major sex appeal, so what? Look at ZSS. She's just as much of a femme fatale. There's a reason her suit somehow contorts perfectly around her tits. Look at Peach, she's got like, boarder line D cups in brawl. So, the sex appeal means nothing negative for her addition. Really, when people complain about most characters, all they're saying is "I don't like that character". Krystal is important ENOUGH, popular ENOUGH, and her moveset is a double entendre. She can be a complete clone, or a staff wielding Dinosaur herder. Plus, Sakurai has a soft spot for SF~ Anyways,
I DO believe there is a certain amount of "importance", AND "popularity" a character needs to get into Smash. You may think that this it's silly to believe that, but really, there IS a reason behind all character's inclusion, besides just playstyle/movesets (Which is undoubtably a reason for a character to be added. There's a reason we have Bowser, not Wart, and there's a reason we got Ice Climbers, not Balloon Fighter. It's not just luck of the draw. Bowser is obviously the more popular, and important choice. With characters like Shiek, and Ice Climbers, they weren't neccisarily about those two things. There addition moreso lands in the "interesting moveset" catagory, IMO. These three reasons aren't the ONLY things Sakurai looks at, but they are something to take into account. For example, we know Sakurai had the idea of adding a retro, and that's part of the reason IC's were chosen. IC's were CHOSEN, because they fit the bill the best. I mean, we all know Balloon Fighter, Urban Champion, Excite Biker, and Bubbles were passed up because they couldn't "do something" (In Sakurai's eyes, NOT MINE)
So yeah, you cannot throw Waluigi out SIMPLY BECAUSE he's a filler character, but it does say something about his chances. As does his obvious popularity, and his AT status.