My boss is an old maid. I don't want to generalize, but all the old maids I know are very irritable. They get annoyed at the smallest and silliest things!

Today, my boss got mad at our coworker because she didn't comb her hair. I think that it's a very small thing that she made into a big deal. Now, she notices every detail!

It's really annoying and frustrating.

How do you deal with a boss like this?

Even though she is nit-picky; she is still the boss, and so keeping her happy and pleased with your work is important for you to keep your job, and also if you want to be promoted.
As for the hair-combing incident; I guess it depends on the working conditions and where it took place.  One job I had was driving a 2-ton hop truck, loaded with hop vines for making beer. No one would have even noticed (or cared if they did notice) whether my hair was brushed or not.
Another  job was working in an office where we dealt with the public constantly; so looking well-groomed was a very important thing in that instance.
So, it is hard to say whether her frustration about the other employee's grooming was deserved or not.