I would be upset too, they probably don’t know how much it means to you. The main thing is your relationship is good, that’s worth more than a card, when I next see them I would jokingly say, is my Mothers Day card still in the post. Wouldn’t make a big deal if it thought and rock a good relationship.,What about a grown up child that never sees you, ever, but sends a card?
Had come to realise ages ago, you cannot compare others relationships to my own, it is what it is, used to be so envious of my friends with close relationships, holidaying together and so forth, all that matters though is that they are living a life they want, are healthy etc. my parents were a big part of mine, with weekly visits and holidays, mine obviously don’t want that.