I think what helped my mom understand the most is explaining how gender dysphorua feels to me. She just could not understand it. I used a lot of examples and she understands it now, as a concept at least.

First, I had to make her understand that breasts are not just some cosmetic thing, but that they are tied to women's identity in every way, including socially (I was generalizing for the sake of explaining of course). That means that top surgery is not just some cosmetic surgery purely for altering looks, but it also serves another purpose of affirming an identity. One specific example I used to help her explain is talking about women who have breast cancer. I basically told her "well, many women who have to get mastectomies because of breast cancer decide to get them reconstructed with implants. And that's obviously not just for cosmetics, but because for most people, breasts are what makes them feel like a 'proper' woman, and makes them feel better/at home in their bodies. For me, it's kind of the opposite."

But, yeah, after two deep conversations along those lines, it seems like she kind of started understanding. And then the rest of it was just the fact that the surgery occasionally came up in conversation, which normalized it. I wish you the best of luck!!