lemmit.online already mirrors Reddit subs and posts.
There were attempts to mirror comments as well but that was awful. It looked like there was a lot of Lemmy activity but it couldn't mirror people's answers back so you were just shouting into a void. And it drowned out all the real discussions because of course the Reddit stuff had a lot more activity.
Every post I saw on lemmit was devoid of comments. With no OP and no other commenters, there was never a reason to see those posts in the first place and I blocked it.
At least it's a good attempt at showing Reddit users that they could move across and not loose anything, getting one user across at a time is the goal with this sort of thing wouldn't it?