I feel as if it would be helpful if your item only appeared in the search ad when more than one of your keywords was searched for. that way, it would eliminate the problem of irrelevant keywords.
for example, i'm doing my harry potter amigurumis, so if someone searches for amigurumi, they're getting harry potter, which they might not care about, but if they were to search for amigurumi + gryffindor, then my search ad would be so much more relevant.
search ads truly need to be refined before they can be of use, and i feel as though there needs to be more communication with us sellers as to where exactly they're going to show up. it seems quite random to me when ads even show up when i'm doing a search.