If you mate with something that has a mutation, it will keep the counter in the child.
Mutation counter doesn't matter once you females with under 20.
Male could have thousands but once the female is under 20 then you can still mutate stats.
Tame a lvl 5 female Rex and clone it as many times then breed your best male against them. If you get a better male then replace old male.
If you get a better female then breed only that female against the male till you get a male with the new stat then breed that male against the clones.