Holm and McGregor haven't proved to be good in wrestling/ bjj. McGregor had some problems with Mendes, so they aren't good at everything and this isn't neccessary. You have to be better than your opponent in certain areas. Four simple examples:
1) Fighter A 'Takedown defense' is better than Fighters B 'take down'; fight will be striking or clinching (Cain vs Dos Santos 2 in which Cain defeated Dos Santos mainly via the clinch)
2) Fighter A takedown defense & 'clinching 'defense' are better than Fighter B takedown & clinching; results in striking (Holly Holm vs Ronda)
3) Fighter B Takedown / wrestling better than Fighter A take down defense / wrestling; fight will likely go to the ground but maybe there is no controll or damage on the ground (Conor vs Mendes)
4) Fighter B Takedown / wrestling and bjj are better than Fighter A takedown defense / wrestling and bjj = fight will likely go to the ground and will be dominated there by fighter B (Maia vs Gunar)
Those examples aren't complete at all, but are just (simple) examples. In the first two examples bjj and wrestling play almost no role in the fight (except for the clinching part). In the last two examples striking will probably be not that important, however all fights start standing, so striking will always be important no matter what, but the fight likely will take place on the ground, so the bjj and wrestling skills will be very important.
And the above makes it clear how important gameplans are. Holly for example maybe hasn't a better takedown defense than Ronda's takedowns, but she and her team exactly knew the strenghts of Ronda.