What about a blank card with a nice picture on the cover? Inside you can write something bland like "Have a happy Mother's Day! -- heart Ally"

There's no lie there about how wonderful she is/was and you can sincerely wish almost anyone a nice day. Alternatively, you're not really obliged to do anything for Mother's Day. But maybe it feels too pointed or confrontational to let it go unmarked? Will the fallout from ignoring it be more trouble than you want to deal with? Good luck with it.