You may not think that a muddy, filthy horse is a pretty sight, but from a horse's point of view, it probably doesn't matter much!
Who hasn't moaned when they've just put their best elbow-grease forward to get their horse looking clean, and that horse has gone and rolled in the nearest muddy patch? Their horse has a good roll, really getting the mud into their mane, all over their face with as much body coverage as they can manage - then they have a quick shake and run off happily, emitting a loud fart in your direction as they run off to show their buddies how great they look!
I love to groom my horses for their (and my) pleasure. I'm probably more practically-inclined than appearance-inclined. This morning groomed 2 of the herd, as their hair was all matted from a couple of days of rain. They appeared to enjoy it, standing still without restraint, now and again indicating where they would like more scratches. End result they were much cleaner, their coat fluffed up again, and I was filthy!
Sometimes I hear from people that their horse 'doesn't like' being groomed. If this is you, ask yourself if you think you are touching your horse in a way that they find pleasurable. Every horse is a little different, and you might have to experiment with different strokes, pressure, brushes etc.
It's not just a matter of 'wham bam thank you mam' when it comes to building the relationship through touch and grooming - touching your horse is an Art. And brushing them is an extension of that Art. (My DVD Touching your Horse with Your Heart explains how to do it.)
If you have to restrain your horse to brush her, if she is trying to bite you, walk off, kick you or any other things that suggest she is not happy for whatever reason, then you owe it to both of you to see what you can do to change the situation. Firstly, it could possibly be soreness, so have your horse looked at by a Professional Equine Chiropractor or other qualified equine therapist. I liken tying up your horse to brush them to tying up your partner to kiss them!
Of course, if you are forgetting that your horse may like being dirty, and welcome a chance to get that way (at least sometimes), that could explain some fidgeting behaviour too! Remember being dirty is natural to a horse and you might be depriving them of some good clean fun by denying them this pleasure!