Basically that's really normal for horses. Most horses if left out on a paddock or not bathed in a long time will have that same build up. It's just a build up of sweat and dirt. Just be thankful you don't own a stallion or a gelding, if you did you would have to do sheath cleaning. Which is basically what you did. The easiest way to do utter cleaning is to use a soft wash cloth or your hand when you're actually bathing and a very gentle soap, like dawn, to actually clean the utter. Make sure you rinse the area really well.

How often you have to clean your mare's utters will depend on how much she rolls and how dirty she gets in that area. Your best bet is to just run your hand down there every time you give her a bath and check. if you don't check the area and don't clean it you run the risk of your horse developing a bean, it's easier for male horses to get but mares will get them to. If your horse get a bean it can make it extremely uncomfortable for them to walk, again worse in geldings and stallions because they won't want to go to the bathroom.