Only Plasma Balls, Sun Rays, Ray of Dooms and special upgrades that negate mirror effect can hit it; Other projectiles defelect off.
The Mirror Bloon is a very difficult bloon to pop. Any sharp projectile, bomb and explosions reflects off it. While this seems bad, the projectiles can pop other bloons after being deflected. To pop it, you must hit it with a Plasma Ball, a Sun Ray or a Ray of Doom/Death Ray. Some upgrades can bypass the mirror even though they are not Plasma, Sun, or destructive beams. Since these bloons are hard to pop they come in rounds 100+. When popped, 4 Ceramic bloons are released. These cannot be Regrowth or Camo or Negative.