I'm kind of with the mother here too. OP, do you know if you have a habit of saying things like this to her which prompted her "you must really think I'm stupid" remark or was this is a one off for both of you?
I ask because my husband and a few members of his family make frequent comments like that. Pull out some leftovers, I get immediately told that it's not a good idea to microwave the leftovers in the container they are being stored in. They say stuff like this so fast they don't see that I'm not intending to do something like that, I'm pulling the leftovers out and going to get a plate. But anyway, I have asked DH several times to stop because it really starts getting too much and at first he will say he is just making conversation. What's not fine is that these aren't conversations I'm interested in having and it does send a signal that he thinks I'm stupid.
No, I do not have a habit of speaking to her as though she doesn't have common sense. My mother happens to be the one who self-defines as not understanding things. She has me sitting in on her meetings with financial consultants and makes self-deprecatory remarks such as "here's another question from the stupid person." I'm the one telling her "you know way more than you give yourself credit for" and participate in the meetings with remarks such as "as my mother mentioned," and techniques to make sure the meeting focuses on her questions and concerns. When she asks whether I agree with what she'd like to do, "well, you have a really great grasp of this, and I think you're asking all the right questions to make the most comfortable decision for yourself, so this seems like a great way for you to go."
As for who has a habit of saying what kind of things, my mother has, for as long as I can remember, said things like "oh, you idiot" to herself whenever she has a minor mishap. Another thing she has said through the decades, to me specifically: "You have to understand, I'm very very sensitive."
So why did I post even asking a question? I interact with people other than her and wanted to get a more general perspective of how my remark (which did use "we," by the way, as is clear in my OP) could be interpreted. The suggestions to frame advice or warnings in the third person, such as using anecdotes about a friend, is a very useful way of depersonalizing the situation, and I thank those of you who suggested it -- as well as other diplomatic wording.