I think tiggytape has a very valid point. I don't think it's the fact that it's a tattoo that's the issue, I think it's the fact that it is the one thing that's an "issue" for her.
A LONG time ago I was with a guy who had long hair. He decided to cut it all off to improve his job prospects, which was a very reasonable thing to do. Trouble is, I have a bit of phobia about hair being cut. It was a really big issue for me, and he did it without any warning. (completely reasonable I know, his hair, his choice.) But it did feel like grieving. It knocked me sidewards, and even though I knew how ridiculous it was, and how utterly irrational, I still couldn't stop bursting into tears and feeling sick.
So I can kind of understand her feeling that way about it. I do think her reaction to her son was unreasonable though. To have laid it all on him, and to have told him their relationship had changed forever, was disgusting behaviour as a parent.