I mean, a passive shield does "mitigate" damage - you directly take less damage than someone without that passive.
Dashing is still a very good way to avoid skillshots and other AoE effects. It's faster than walking. Increases the likelihood of dodging some damage. As annoying as any other character with a dash. It's limitations are countered by several other factors, including ramping damage and a short cooldown.
Low base HP is expected when you have a passive shield and other survivability tools (see also: Morde and Nasus)
Riot has mentioned that all champs will soon be getting scaling MR as an additional balance lever, so this will soon be irrelevant. Also, I believe that several other melee champs, such as Yi and Kat, lack scaling MR. Almost all ranged champs lack scaling MR.