MIL has never liked the holidays, and has never been a gift giver. The most I remember her giving is a can of coffee. She is also not a graceful gift receiver. Nothing is ever to her liking. That was ok because we did not see her on Christmas. Now that she is living with us, she has voiced several times her disdain at decorating, trees, etc. . . We plan to have a big meal with friends coming over. How do we have a happy Christmas without her sucking the fun out of it?
I suggest you go about your decorating, trees, etc. It is your house. Tell her you wouldn't dream of telling her she must decorate her room. Perhaps there is one other area you could leave "plain" for her comfort. Don't get mad, and don't let her suck the fun out of it for you. Give her choices. "We do our gift exchange on Christmas Eve. You are very welcome to participate, but if you prefer not to, that is OK, too." "Our friends are coming over for a big holiday meal on Christmas Day. Would you like to be included, or do you want to make other plans?"
If I took my son's grumbling to heart, it would suck the fun out of my holiday. It may annoy me for the few minutes he is doing it, but I don't let it get me down. He is entitled to his own attitudes, but my decisions prevail in my house.