As a fan of the X-Films, the only issue I have with the argument of Vanessa/Copycat as mutant or not...we're talking about series where characters have been, lets say revised, over the course of the X-Films.
I think so many more prominent characters in the grand scheme of the X-Universe and mythos have been featured on film, whether Vanessa is mutant or not, she's just so minuscule, at least to me, its not important, and the franchise, again is known for changing characters to suit the stories being told on film.
For example:
--Deathstryke wasn't a cyborg...she was essentially a Wolverine clone...
--Juggernaut wasn't a mystically endowed human...he was a mutant...
--Callisto was given a grab bag of other characters abilities...that would either show up, or are set to show up in the films...
--Silverfox isn't a mutant with mind control (maybe pheromone? or whatever from Origins)...
--Viper's reptilian qualities...
I could probably list a few more but they're escaping me...
The X-Franchise and related films have a history of altering characters (so does the MCU, so I don't want to be attacked for either, or, okay...) but for the X-Franchise, except for the core/main cast, they meddle with the smaller roles, so I don't get the Vanessa must be a mutant?