...so buy your logic the husband should have gone to Starbucks and bought a voucher for his mum and then the OP should have walked in 2 minutes later and bought the one for her mum.
Or the one who happened to be there at the time (ie the OP) buys both at the same time.
If he had been the one to go to Starbucks and buy the 2 vouchers - one of which is for his mil (ie the OPs mum) - would people be saying "oh that's terrible, how dare she not get her OWN mother a gift, getting her husband to do it, how lazy etc".
OK, let's explain this step by step.
A grown man whose wife has just had a baby buys his own mother a gift for Mother's Day. This saves his wife the bother of adding yet one more thing to her already hard-working-on-little-sleep-post-partum brain.
Now - and this is where being a grown up comes in - are you ready?
He does this, if Mother's Day gifts are something he considers important, every single year, regardless of whether his wife has just had a baby.
Obviously, if he is ill, or if there is some reason he can't put his mind to the gift for his own mother, he could ask his wife to help him out. Maybe she could pick up something he has ordered or bought. Maybe she could wrap it. He would need to do the work of planning it and ordering it all by himself, and he would need to ask his wife for her help in the spirit of someone asking a big favour, if he needed help.
Assuming the person you are married to will take over all your life admin is entitled behaviour and grown ups should avoid it.
This is called adulting. Everyone should try it.
A grown woman does the same wrt a gift for her own mother, if that is what she wishes to do to honour her mother.
Or in this particular instance, if her husband was going out to Starbucks to buy his mother a card and he was aware that she had just had a baby and it might be difficult for her to get out (you never know, some things are not that clear to some men) he could ask her if he could pick up a card for her mother while he was at it, or had she already arranged something.
Again, adulting...
I really don't know why it's so hard for posters here to understand why each grown up should do their own life admin, buy gifts for their own parents.
It's almost as if the concept of a woman's right not to be saddled with managing her husband's family relationships on his behalf hasn't occurred to some people.