It's more about movement and such, Dracula is still a big immobile boss who's every movement is telegraphed, Skull Kid on the other hand is smaller than most of the characters and would look very unnatural just standing still and not flinching while you beat the snot out of him, he would only make sense if he were to dart around the room and be highly mobile, but with such a small character that becomes a huge pain and an awful boss fight at its very core.
A boss having a weak point is no issue at all, it's very common in sidescroller games like Metroid, Mega Man, and occasionally Kirby, but having a small frame is, both thematically and in terms of gameplay. There's a reason that the smallest bosses in those games are at worst level with the protagonist, and that's usually to hammer down some sort of rivalry theme (Meta Knight, Proto Man/Robot Masters, SA-X).
As I've said before, I really don't think he'll be a boss, doesn't mean he'll be playable either mind you, but it seems like the only thought behind the idea of him being a boss is "He's a bad guy, so he must be a boss" when it isn't that simple. All of the rules about nailing the personality of a character also apply to bosses, and I just don't see it being possible with Skull Kid within Smash's framework.