Obviously depends how you look at it. Nick Diaz gives his own pov; https://www.youtube.com/watch?v=qPOOzufOpcA
MMA = decathlon Boxing = 100m MMA requires a wider variety of skills, which couldn't possibly be as refined as one single skill. Boxers should easily outbox MMA fighters. MMA fighters should probably beat boxers in an MMA match, because they are allowed to use so many different techniques. MMA is still in relative infancy compared with boxing, so some boxers could do well just boxing and learning a few techniques to avoid getting taken to the ground. Eventually the best MMA fighters will be very good at most of the techniques and probably have a favourite technique, e.g. striking, kicking, wrestling.