Are thieves at a disadvantage when it comes to dragon fights?
Solution 1:
I'm a Thief with a bow... Archer Thief. I have no real issues, just have to have good cover nearby!
Solution 2:
Earlier on you'll have to be a total wimp and kite the dragon around, plinking at it from cover with your bow. Be sure to keep lots of ammo.
Later on, I'm currently level 25 and I've assassinated a couple of slumbering dragons with one well placed backstab(with a dagger, of course). Just make sure your Sneak and 1-hander skill is solid if you wanna try that though.