You have to remember that Mantra is a cross class ability and it's balanced as such.
Everyone has access to the 5% version, Mnks bring the 20%.

Also, I don't really agree about Mantra being very small in range. It's fairly big compared to, say Cure III's aoe heal (where you really do need to be almost touching).
In most situations where Mantra is good, you are stacking for AoE Damage anyway (Titan's Geocrush or Stomps, Garuda's AoE, Turn 4's burn phase, Turn 5's conflags etc.).
In other cases, you may use it as a Tank CD which isn't that hard since you're melee anyway.

Ifrit has a pretty big arena so it's not the best judge. If people stacked after the nails for a SCH's Sacred Soil, then they would likewise get hit by Mantra. I would say it's the same range, and that's a decent size.