You could still be a nice person. Congratulating on holidays or otherwise keeping contact with exes or soon to be exes doesn’t really fall into a category of being “nice”, it isn’t required.

I’d say contacting ex or responding to their contacts when there is death or illness in a family would fall into a category of being nice, but even that isn’t required when it comes to exes, just a kind gesture.

Holidays isn’t an emergency or life altering event that you need to show your compassion or something. Not fussing over him on holidays won’t make you “not nice” at all