I believe the more common acceptanc

I believe the more common acceptance of "Cut the mustard" comes from "cutting" or diluting mustard with vinegar. By cutting mustard with vinegar, it makes it taste better. So, someone that was very good at "cutting the mustard" was regarded as skilled. But if someone was not as skilled at the proper dilution of vinegar to mustard, it was said, "he couldn't cut the mustard".