She's being a brat. I would let it die but if she brings it up again he should say "I sent you flowers and sent you a message first thing and made plans to come see you as soon as I got home and invited you for a skype chat, which you ignored. You know that phone calls are so expensive that they're on an "emergency only" basis. I didn't even call my wife!"
I hope he's not sitting somewhere feeling guilty, because he shouldn't!
JenJay's response really resonated with me, especially that OP and DH should not
be feeling guilty over this!
I think it is fair, since MIL has made her disappointment known, to ask what exactly her expectations are for Mother's Day and how OP&DH failed to meet them with the things they did and the things they offered. "This way, we can avoid repeating the same problem in the future."
If she's like my mother, she will say exactly what bothered her about the day-- like she wanted to be remembered by her son first thing in the morning with a phone call to show for it even if it was expensive, rather than a pre-scheduled way around the celebration. (Please note, OP: those aren't my feelings on what you and DH did; I believe you were very thoughtful!) Then she'll feel bad for being upset about such a silly little thing. Whatever honest answer she gives you will at least give you something to work with.
If she's like my grandmother, you'll never get a straight answer as she'll say she needs nothing, she's not that fussy, she wasn't really that upset on Mother's Day, you all are overreacting... Well, no big deal then, we'll remember that next year! And then just try to let the incident fade away into the past.