Got an Answer?
I don't have any advice on how to get her to stop. With my kids I just smacked them and made a big deal out of how naughty and disgusting it is. But that's not for everyone. My advice to help keep your cool is to think of it like, she's trying to paint you a beautiful picture. She doesn't realise how gross it is she just sees it as trying to make you happy with her beautiful artwork.
Maybe she could have some washable markers and paper in her room when she goes down for her nap? (Use the ultra washable ones, they're amazing! They come out with plain water, of everything I've encountered so far)
Maybe she is ready for big girl pants? We don't wanna get poppy on our super girl or pretty princesses.
Is she still in nappies? Or toilet training? If in nappies, try putting the nappy on backwards. Or if toilet training, make sure there is a potty near her at all times. And just explain that poo is for the toilet, not to play with.