Well if you look at the mutation chart listed in the hatchery, you'll come up with this:
- 17 out of 1000 fresh eggs will hatch mutants
- 24 out of 1000 old eggs will hatch mutants
- 35 out of 1000 spoiled eggs will hatch mutants
These are the numbers, but it ultimately depends on luck I guess :D
If you're only looking for mutants and care less about hatching any other pet, spoiled is the way to go, because even though there's a very low chance in an actual hatch, you're almost guaranteed that it's a mutant. And just my 2 cents on old eggs... since not even half of the eggs will even hatch and the mutation chance is very low, I wouldn't even bother with those and either sell or wait until they're spoiled.