FormerlyHemlock
Hero
The issue I see is that there are a lot of AoE attacks in 5E. What good is a defensive spell that can be totally dispelled and have zero net defensive effect because some hedge wizard has Thunderwave or some Shaman has Spike Growth? Casting a defensive spell in 5E is kind of like casting a Healing spell. The caster is already at a bit of a disadvantage because offense typically trumps defense in 5E (with some rare exceptions like Greater Invisibility).
Magic Missile or Scorching Ray is fine because the defensive spell actually does something defensive. Each image popped prevented the Mage from getting popped.
Without getting sidetracked into a discussion of the relative merits of offensive vs. defensive tactics in 5E, suffice to say that Paper disproves Spock and Spock vaporizes Rock: (Rewritten) Mirror Image doesn't have to be strong against everything to be a worthwhile spell. It's great against Iron Golems and Disintegrate and Counterspell--it will save you somewhere between 60 and 200 HP against Iron Golems and Disintegrate--and it is poor against Fireball and Hypnotic Pattern.
If it were great against Fireball and Hypnotic Pattern too, or even just ignored them, it would be (IMO) inappropriate as a 2nd level spell. It is better and more interesting if Thunderwave functions as a counter to Mirror Image. Besides--the hedge wizard is still wasting his action running up into melee range and casting Thunderwave, which invites its own counterplay (kill him).
But, I've acknowledged my bias--I'm probably being influenced by aesthetic concerns and old memories (and modern games like Dominions 4). And I'm not actually planning on houseruling Mirror Image any time soon anyway--no one ever casts it anyway, so fixing the spell's logic isn't a high priority for me right now. YMMV.