Depends. I came out to my parents after a childish tantrum on my mother's part because I literally didn't want to speak to her again. I wanted her to know I'd had these problems for decades which she'd been actively waving away as if nothing and I wanted to do it there and then before she decided never to speak to me again. A final word, so to speak, and I was done with her tbh. It didn't get the effect I hoped for, though... she acted as if I was talking nonsense, made no effort to discuss it except over a couple of emails and then barely spoke to me at all for 2 years due to continued pouty stropping about the original disagreement. We live 100s of miles from each other so there's that, but there was no effort after the "revelation" to actually talk to me about it or acknowledge it properly. It was only finally brought up as a serious topic following the coming out of my cousin 2 yrs later, which didn't go well either.
If you're angry enough not to care about your mother's reaction - which might not be good - maybe you've nothing to lose. If you're going to be around her and dealing with her and if you want to foster some positive relationship though, probably best to sort out your issues first or the trans issue may well just be brushed aside or not taken seriously. Even at the best of times it's not taken seriously by some people's parents, I think it's best to be in a serious and neutral state of affairs before discussing it with them.
