Depends on the niche. I do not sell shoes
but I understand that in that niche reversals are always in the 30-40% range. In my primary areas of home and garden products, the normal rate is under 10% (most merchants less than 5%) and I would question keeping any merchant that exceeds that for more than a brief period.
More important is why are sales being reversed. No reason or meaningless terminology is bad, bad credit card is acceptable, a specific problem with a specific product is ok, but repeated reversals because products are out of stock or otherwise unavailable are totally unacceptable.
I very rarely have had reversals due to tracking issues, such as "wrong referral" or similar, but if I saw that happening, I would inquire and if no satisfactory response was received, I would likely drop the merchant.