Mysterio being a shapeshifter kinda ruins his gravitas anyway. If we are meant to believe he's some sort of supernatural for a large portion of the movie, then whats the point of making him wear a disguise?
Mysterio hasn't always been depicted as being able to shapeshift or mimic appearances, it's often depended on the writer. If Chameleon is indeed in this, then he could fill that role.
Neither one is technically a shapeshifter, if I'm not mistaken.