Having more experience doesn't neccesarily translate into a better candidate. Lebron james at age 18 was already better than most of the veteran of the NBA, even veterans at their peak athletic age. The fact that Mystique has fought longer than DD in a chronological sense doesn't mean she is a better fighter.
Actually its not pure fandom to say DD is a better fighter than Mystique. As readers all we have to go by is what has transpired in the comics. If Mystique has not performed any feats which show that she is above DD when it comes to h2h fighting then we can only assume DD is better. To say that Mystique is actually a better fighter than has been proven in the books is more of fanboydom. Mystique will be able to hold her own in the fight due to ability to shape-shift, but she can't change into anything that DD can't handle. His senses will be to much for her to handle