I really think whatever a priest does is going to make someone upset.
Some priests will bless the mothers, but then some will feel left out or hurt for many of the reasons already listed.
I’ve seen some priests try to solve this by asking all the women to stand up to honor them as mothers, grandmothers, godmothers, spiritual mothers, aunts, sisters, friends, etc. But then some will get upset because it’s Mother’s Day, not Women’s Day. And some women will feel awkward standing for a blessing on Mother’s Day when they are not a mother, yet feel awkward not standing up when the priest has asked them to do so.
And if the priest doesn’t acknowledge it at all, then he’s ignoring the importance of mothers.
It’s basically a no-win situation.