Self-defence is too wide of a subject area, MMA covers some areas but not others.
Like if I can play guitar, and say the electric guitar is "good" or even "best" for music. Then get told to play Mozart... maybe not the best for that.
Self-defence is mostly non-physical, but ignoring that aspect, there are still many different situations:
weapons, MMA is not the best for this.
Someone is being a pain, physical control is needed, just to get that pain compliance but not neccessarily escalate to a fight, again, MMA is not the best for this.
Anything other then a one on one? Might work ok, but most never train it..
One on one fight? This is where MMA shines, but it is only one aspect of self-defence, and for most people not at the top of the list of likelyness to be needed.
MMA is best used for fun, fitness, sport, social activity, and just cause you love it. Self-defence aspects are side-effects, not goals.