Lets get a few questions out of the way:
Q: Are these Christmas cards or holiday cards?
A: These are Christmas cards.
Q: Is it a micro-aggression to call these Christmas cards?
A: No. Because Mike said so, and because there is not much reference to Christ in these cards. (Take a look for yourself.)
Q: Then why call them Christmas cards?
A: Because it is more efficient than calling them "This is how Mike stops during a happy time of year to show he is paying attention and he cares."