Well - given that I don't know what the trigger comment was, like most of the other posters, it's difficult for me to And though I know I'm stepping into potential flame territory from the "just get over it" folks - I'll comment, I'm in OPs wife's shoes.
I am a DIL who no longer sees my in-laws, nor have I seen them for I guess about two and half years now. But for me, it wasn't one single comment. It was a series of them from the first time we met. Every single time I was around her, she said something or did something incredibly hurtful and disrespectful - which while some might seem quite petty to a casual observer, they all combined to make it painfully clear that I was not welcome or accepted. That I indeed was not part of the family, that I was only begrudgingly allowed to be around.
Which for me was compounded by the fact that when these comments were made, my husband never said anything. And thus not only did I feel shunned and embarrassed by the public ridicule, but then abandoned to fend for myself as well. If and when we have discussed it, I got the "she can't help it, it's just the way she is" thing - telling me he saw nothing wrong with it. Or - that it didn't matter it was wrong - she had an excuse.
Which - if your wife is angry at you in connection with your mother's comments, it's possible that she has the same feelings. Which yes, not visiting is supporting her in one way, but - it's different. It's different than supporting her in the moment. So how did you react when this original exchange was made from your mother to your wife? How did everyone else react? Was this done in front of your entire family? When she no doubt talked to you about it after the fact, what did you say? Did you tell her "it's not that big a deal" or "it's just how she is" - which could be interpreted as dismissing her feelings as invalid.
Can't blame my dad for sticking up for his woman... as I'd do the same if I were him.
So - did you do the same? What was your participation in this original row? Which you might not want to get into either, but - this might be what spoke volumes to your wife more than what your mother said. It was how you were or weren't going to support her in their presence. That it caused a breech in trust between you - that she couldn't trust for you to "have her back." Which isn't something that is easily "just gotten over."
Without more information though, it is indeed difficult to surmise, that's one possible string of thought though.