I told her it would be better if she'd just said, "I bought some decorations, and I think you'll like them." instead of saying she bought a "gift for" me. She thinks I'm just being ornery, and she was upset that I didn't get excited about the stuff.

NTA Your wife crossed the line into being unreasonable by expecting you to display excitement over something that she is into that she already knows you aren't. I think you are right in that if she had phrased it in the way you suggest she would have gotten a better reaction from you.

She may be just frustrated that she can't share her joy in decorating with you. This "gift" was really about her wanting to share the happiness of her interests with you and get you interested, not in getting you a gift that was just about what she knows would make you happy.