Sometimes they are very messy if they let go a really big broody poo after holding for 2 days during the final days of lock down.
I do try not to disturb my mommas those first few days/week as she really needs to bond with her babies, and you can definitely interrupt that.
Clean her if really she really needs to be, usually by doing some judicial trimming of feathers. However, honestly don't worry too much. I am amazed how in a few days with dust bathing most of it knocks off. (Just make sure she isn't sick...but chances are it was a big load of broody poo).
She'll also molt soon and have new clean feathers. I figure it is Nature's way of cleaning up the mess after brooding and having the babes run (and poo) in your hair...er...feathers. So glad I am not a chicken mom....oh wait...during those baby years I wore a lot of spit up and poo too.