Personally, I don't really care what a coach is wearing, either. I'd prefer them to be clean and neat, same as any other person I have to deal with in any other walk of life. Jeans wouldn't bother me. Well, they wouldn't bother me on someone else. I grew up with the same aversion for jeans on a baseball or softball field and, to me, they seem out of place. But that's just me. I couldn't care less if a coach was wearing them.
This falls into the same category as a lot of other stuff umpires are forced to deal with that doesn't have much at all to do with playing the game. Besides coaching attire, there are rules for player uniforms, jewelry, the color of their gloves and even what they can use to tie up their hair. I hate dealing with this stuff. I think that it makes the umpires come off as too hardnosed and inflexible when they have to crack down on coaches and players for these things.
BUT...if you're there getting paid to umpire a game, you have to enforce all the rules, not just the ones you like. You can't just ignore the ones that you don't agree with.