I would suggest each time she is with you and dad, you all talk to daughter about how she can clean her own body and hair (this mostly in the future) .
Give her 2 really cute plastic buckets or holder and place in them cute wash cloths, soap, toothbrush, tooth paste, shampoo. And tell her one is for her home with you all and one if for her home with mom.
I recall as a child, my mother had the same routine for baths every time. SO when it was my time to bathe myself, I naturally followed her routine. When I was ready for my hair to be washed and back scrubbed, I would call her in (I had super long 1960's straight hair").
She would always start with washing my hair, then my face, then my neck and ears, then my trunk, she would do my back, then my bottom, then legs, feet (remind her about between the toes). . Start from the top and go to your feet. So one night you totally bather her, the next night let her do it, but you sit there and guide her and praise her.
Remind her, that now that it is getting warmer, she will need to bathe every night. If her mom will fill the tub, daughter can pretty much wash herself.
Help her with wiping. This is one of the hardest things to teach children. They have a hard time reaching, so work on this with her. Wipe until there is on more poop on the paper "if you need help" "ask me or dad to help you".
A child that is not well cared for, shows signs of a depressed, over worked or distracted parent. (Or, she is doing this on purpose to upset you and the ex) I know you are concerned. I would document these incidents (do not make it a big deal, do not tell her in case this is just a momentary lapse).
It is ok every once in a while for this to maybe going on, but if it is truly every week, your husband may need to ask his ex if everything is ok? Maybe she needs a break?
Decide if you are willing to keep this child for a few weeks. Make it an offer, because you are concerned, not a threat with judgement.
Try to be calm about this, no drama for the mama!