Like geek says, you definitely can't beat yourself up over not doing decorations that one christmas. Teens will be teens, and your mother surely knew that. She may not have seen the decorations that year but she saw you being you, a growing girl with all of the things that come with that. Don't think that wasn't special to her, even if you were a grumpy teen like the rest of us.
And honestly, even if you weren't a teen when she died, and even if you had put up the decorations for her last xmas, I do think most people will find something/anything to hold in their heart as regret when they lose a loved one, even if it's not fair on themselves to do so. I know I still don't feel 100% right about not visiting my dad for his last two weeks, even though it was a rational decision I made at the time for his sake and mine. For some reason it still feels wrong, like I should have been there, even if he had no clue who I was, even if my presence freaked him out or confused him, even if it was murder to see him like that, and even though I knew, if he was lucid, that he'd be devastated to know that I had seen him like that. It just feels wrong.
I must admit I feel I'm missing the insight into my dad's life that I'd maybe have had if he was still around. The good news is, I'm more close to my mum since he passed than I think I'd ever have been otherwise. And I feel we can talk about anything, and that's amazing. I kind of wish I could have had that with my dad to the same extent.
But it's nice that my mum can tell me things about my dad's past, at least about how he was a guy in his early 20s when they met. A player of a guy, pissed around every girl he had because he knew he could, and then my mum came along and got played too but played him right back. And then it was love.
I adore those little bits of insight. Even the stuff that shows him in a bad light.