I would say dag her if she's mucky, do the 'wheel arch' bit so her lambs can get to the teats but otherwise leave her for 2 or 3 weeks more to let the lambs grow a bit more first. Here in southern Scotland we do our earliest shearing in May, but that's the tups - ewes are done mostly in June . Tatty doesn't matter, unless she is dirty and attracting flies. Once she is shorn, if you can, pen her with her lambs for half an hour so they get to know her again and don't miss out on a feed.