Well, it depends.
If the priest does not intend to deviate from the rubrics, no harm, no foul.
If the priest does intend to deviate from the rubrics, why? By doing that, he essentially says “I know better than the Magisterium, instituted by Christ.” So, while God does not view the Masses themselves as any less of a Mass, the priest disrespects the rightful authority of the Church.