According to my games (FFS and DS, older player/started games fairly early-ish but went on hiatus), Ultra Rares can be more common than Super Rares and even Rares and I think at least some of this boils down to RNG.
In DS, I was able to breed 3 Diamonds and not a single Rare or Super Rare Event Dragon in the span of about 3 months of trying between October to December. The Diamonds were the result of me desperately tossing all colors together in hopes of ANY new Rare or Super Rare Dragon in the last month of trying for Limiteds with zero success (and using the targeted colors only; four colors were used only after 2 months had passed without success using targeted colors).
I leveled to fifteen in FFS and in my frustration to breed a Limited (there's a pattern here!) or just something ANYTHING new for my park to finish some goals, I tossed together four colors and after popping out a few Commons (Ferrets and Armordillos as always), I suddenly got a Crystal Unicorn.
It's DS all over again for me (if I kept every Common I bred, I would have a park full of Armordillos, Ferrets, Rainguins, and Turtisles and little else) which makes me think that to some degree, individual RNG plays a role in all of this.
I quit FFS some months ago because of this breeding problem - Commons everywhere, a few Rares, and LOTS of duplicate Rares and little variety regardless of pairings - and when I came back to try the Valentine event, I got results similar to the results I got in DS - one random 44 hour Ultra Rare during an Event that clogged my game for four days.