Yes, stifle problems are hereditary. Rather, not so much "stifle problems" themselves, but for sure the poor stifle conformation that leads to stifle problems--yes, certainly that poor conformation is hereditary.
As to whether it comes from the mare or the stallion or both--it can come from either, or both. The foal can inherit a poor stifle from one parent--someone might breed a mare with a weak stifle to a stallion with a strong stifle in hopes that the foal will take after the sire. However, the foal can just as easily take after the mare, and have her weak stifle (though yes, the foal could get lucky & take after its sire, and have a good stifle.) And of course if you breed weak stifle to weak stifle, chances are much better that the resulting foal will have poor stifle conformation.
More people need to educate themselves as to what makes a stifle strong or weak, and take a serious look at their breeding stock, and weed out those with poor stifle conformation. Too many people convince themselves that stifle problems are just growth issues, or injury related, and just continue to breed for poor stifles.