Hmm, perhaps it's the outline that causes the EM's sprite to stand out a bit. If you look at the other examples, the light source is coming from the top left in each, as such, the Pokemon only have a solid black outline on the bottom right of their bodies. This is especially noticeable on Claydol and Garchomp where the lines that are exposed to the light are simply a lighter shade of the shape's color.
Sprite's still looking great, Wyverii, but I don't know about the sludge. Every Poison Pokemon's sprite other than Koffing/Weezing get by without any overtly poisonous 'props'. I don't think we need to beat over anyone's head that this is a Poison Pokemon when historically we've accepted it simply because Nintendo tells us to.