Okay so lets analyse the champs in each division to determine whether more of them are tall or short for their divisions.
HW-Stipe-6ft 4-Normal height
LHW-Jones-6 ft 4-Tall
MW- Adesanya- 6ft 4- Tall
WW- Usman- 6ft- Taller Average
LW- Khabib- 5ft 10- Average
FW- Volkanoski- 5ft 6- short
BW- Cejudo -5ft 3- short
Notice that it is only in the smaller weight classes where being short seems to not matter. In all the other weight classes the best are either normal sized or tall. So the answer really depends on weight class. Overall though being tall won more times than being short, and being short only worked when the primary skill set of the fighter in question was wrestling.
That's just a snapshot in time, though.
It conveniently sidesteps DC and Fedor, two of the all time absolute beasts. As well as Hughes and GSP... and when GSP finally met someone who gave him a real go, it was the shorter Hendricks. Hendo was pretty average height for the classes he fought, and won championships in. Penn, at his best, beat up taller guys on the reg, and Sherk has a much better record than I think a lot of people recognize. MW champions have been mostly taller guys, but GSP still went up, after a long layoff and well out of his prime, and snatched the belt, and Rashad, a natural MW if ever there was one, was pretty beastly for a while, at LHW.
I do agree that you pretty much need to be a wrestler to be effective if you are shorter than average. I would add that you also need to have a great chin.
But let's not pretend it's crazy rare just because Jon Jones exists and Adesanya and Usman happen to be (very new and unproven) champions at this present moment.
More recently, Jon Jones does skew the overall numbers, but nevertheless, height is not a very good predictor of fight outcomes. If you put $10,000 away to make small bets based on height, you would bleed money until it was gone. Every cent you made on Jones you would lose on DC... well... until they met each other, I suppose.
Statistically, reach is far more important (and it doesn't match up with height as closely as you might think) and AGE is by far the best predictor of fight outcomes (especially rematches... hard to even think of a younger fighter who has won the first fight who has ever lost the rematch).