a common complaint as far as i can tell; dragons arent just easy, theyre easy compared to every other enemy. because theyre flying most of the time, i guess bethesda felt it unfair to make to make their fire/ frost breath too damaging, and thus dragons barely scratch you.
All depends on what perks/gear/stats/enchantments you have, and the levels that you run into certain dragons. My first frost dragon was a Blood Dragon that I ran into in the high teens/low 20's. One of the ones tied to a dragon wall.
It's frost breath utterly destroyed me. 6-8 levels later, more HP, doing more attack damage, better gear, and with the "resist elemental damage" Block Perk? It was a lot easier.