There's no blanket right or wrong answer to the question "should a stepmum get a card on mother's day" Blended families are all so very different. What's fair, understandable or reasonable in one situation is different in another situation.
In my house alone we have three different situations where the card/gift on Mother's Day is very different.
DS1 is my DSS technically. His Mummy died when he was a toddler. He took to calling me Mum when he was around 9 and it was his choice and suggestion. He was the first to suggest he wanted to buy two bunches of flowers when DH took him to buy flowers to take to his Mummy's grave.
My girls have experienced two step-mother figures from their Dad over the years. His wife, who he recently split from, was originally lovely if not a little over-bearing, however as soon as she had their first half-sibling unfriendly, unwelcoming, and on various occasions downright cruel. She frequently bitched to them about how much maintenance their father paid (the minimum), told them they couldn't attend their half-brother's Christening because it was for "close family", frequently "accidentally" booked things that didn't include them for their contact time and actually told one of them it would be better if she left school and got a job because staying on was going to be very expensive for her Dad. Most recently, after splitting with their Dad, she pretended she wanted the girls to spend time with their half-siblings on a set night a week. When the girls had to change the timings after a few weeks because the bus timings changed she called them selfish little bitches and banned them from seeing their siblings. Turned out she had a class booked and was simply using them as free babysitters.
However, they do still send a Mothering Sunday card to his ex partner because she was lovely to them when they were young. She was the kind of step-Mum who made them dinner, wiped their tears and helped with their homework. They've never forgotten her kindness and they still keep in touch. They call her child their "sort-of-half sibling" and she's a lovely lady.