I think the snakes work somewhat well. From what I understand, in NINJAGO, the Serpentine are about as numerous and important as humans. Therefore, it makes sense to have both humans and serpentine prominent. Humans and Serpentine did not have the best relations with each other, explaining why they come in opposition to one another. And it is not like NINJAGO uses only Humans and Serpentine as its characters. There are the Dragons, which are very prominent, and other species like say the Djinn and Oni.
But it is true, that while it adds cohesivness to the universe, it can be sometimes too much, and having more creativity is certainly a good thing. Vermillion are to me well done variation on standard Serpentine. They are not Serpentine proper, they are a variation, like Skulkin were of humans, more interesting than to have just another standard Serpentine.