No, you are not being unreasonable.
My guess is that she has always been critical as well of you and it is not your fault she is like this. Did your children hear her say that to you?. What do they think of their nan being so critical?.
I guess too that to date your mother has not apologised nor even accepted any responsibility for her actions.
Honestly, if she cannot or will not behave then you and your family do not see her. A 2.5 hour trip to be insulted 5 minutes in, sod that. You would not have tolerated that from a friend, family are no different really. I would reassess your whole relationship with your mother, look closely at what she has been really like as a parent till now.
I would not let your Dad entirely off the hook either; he may well be a bystander to all this, a man who acts out of self preservation and want of a quiet life. He presumably did not say anything to his wife in response to making such a comment to you.
BTW do you have siblings, if so what is she like with them?.
What you need to do is have proper boundaries in place re your mother as to what is and what is not acceptable from her. Saying, "did you mean to be so rude" at the time as a retort is just a temporary fix to a long term problem. It basically does not work other than that.
Also, you do not need your mother's approval any more.