I would interpret the "Dear Son & His Wife" also meaning "Dear D-I-L". One need not write "Dear Son & Dear D-I-L" to convey that meaning. I wouldn't read anything negative into what the card said. I think your m-i-l likely just liked the picture on the card and/or what was written inside.

I also sometimes have trouble finding a good greeting card. Sometimes I buy the blank ones and write something inside, myself. My husband and I have a tradition of mostly only buying each other cards that have a bird(s) on the cover. That obviously limits options. There have been times when we bought each other the same exact card. LOL!