She will eventually. You could put about an inch of warm water in a sink, and then holding her firmly, gently lower he rear end and feet into the water while supporting her body and upper paws. If one of you holds her that way the other could take a mildly soapy washcloth (like baby soap – fragrance free) and wipe her off down there. Towel her dry and never get her face or ears wet even if she struggles and gets her front paws or tummy in the water.
Another completely dry method is to take some plain corn starch (like you get for cooking) and dust her butt area all over with it. Let it stick, then after a while it will dry, soaking up any sticky moist areas, then you can use a towel or paper towel to gently remove it all off of her.
What smells are her mushed cecals. I assume since you know it’s poopy butt that you’ve checked with your vet to be sure it’s not an illness? If it lasts more than a day and you haven’t, get her seen by your vet.