My horse is exactly the same. I feel like tearing my hair out when i've spent an hour or so grooming her and she comes in from the field covered in mud & I think 'You're supposed to be a lady!'.
I've found that plastic curry combs so wonders to get off solid mud but they do leave dust residue so after you've got off the dried mud clumps go over the area with a flick brush to remove surface dust, then a body brush to remove underlying dust and make the coat gleaming again (Well, until next time!)
Yeah, my mom's tobiano gelding is a pain to keep white! Normally, though, the mud falls off by itself, or he will roll in the dry dirt, but when he looks horrible in the morning, he looks better by afternoon. Though our horses are pastured 24/7 with a walkin barn anytime they want.
We've had 40 inches of rain over our normal year so far and it's still only November. So... my guys seem like they're always muddy. In the summer it seems to help them keep the flys and mosquitoes off but I worry as it gets colder that their hair won't be as insulating as it needs to be. Some of our mud is that red clay that is really sticky and dries like concrete. I just do the best I can and realize that they're going to be a little muddy sometimes. I just always try to get them as clean as possible in the saddle and girt area before I ride. My horses are pastured 24/7 as well. And of course the one that gets the muddiest is Keystone my light gray!