That's not a good reason why he can't be a Dark type. There are plenty of examples in Pokemon that show that thinking with looks does not work.
Dragons are thought to be scary looking, brutal killing creatures. However, the entire Dragonite line is anything but that. You also have Altaria which looks like it would rather kill you with love. Then you have Axew, another cute Dragon type. Most of the Dragon type, save a few, don't even fit the description of scary, brutal killers.
You would also think Ghost types to be scary, but there are a few that aren't such as Drifloon, Misdreavus, and Shedinja.
Judging what type a Pokemon will turn into based on how it looks is kinda stupid imo. I mean, who honestly thought Torchic would turn into a Fire/Fighting? It's a tiny chicken, there's no way it could get that typing later on, right? There's also other things to consider like how Pokemon can drastically change in appearance and personality when they evolve. Just look at Feebas and Magikarp. One went from the ugliest Pokemon to the most beautiful, and the other went from the most pathetic to one of the most seriously badass Pokemon we know of today.
Chespin for Dark type.