The problem with a caesarian is the scar tissue on the womb means it does not stretch as well as it used to, either during pregnancy or during birth- I can see no reason why fertility would be affected- it never has been in cats or dogs I have bred from, nor in any of the mares I know of that have been bred after this op.
I have never, Thank God had to have this done myself but I have always said that, were I to, I would not breed from the mare again- enough things can go wrong without added complications.
However, if you have a very good Vet facility near you, and they would be willing to have the mare in to foal her down the next time around, and you are willing to take the chance on losing the mare (always a chance whenever you put a mare in foal, anyway) and you really do need a foal from this mare, I would wait a while and then get a full Vet rundown and take their advice on the matter.