I've tried to keep a balance on the site by only creating specific categories for the popular to very popular EV models. Up until now, the Smart ED wasn't that relevant in the EV market and was a relatively old model. It didn't make sense to have a separate category for it as there wouldn't be enough discussions on this forum to warrant it.
However, that was until Smart announced the new version for 2017. I expect that model will sell like hot cakes.
I think it would be a little premature to add a top-level category for Smart but I will gladly add a Smart subcategory to the #other-electric-vehicles category. Is this what you were after?