Mods which give an unfair advantage are not allowed.
"The use of hacked clients it’s absolutely forbidden since they give a big advantage to the player using them and we believe this ruins the Gameplay
The Optifine mod is allowed since some of our users can’t play without it. Other mods such as MCcapes, Rei’s mini-map (without entity detector) etc… are allowed since they don’t give a big advantage…"
So, no. Damage Indicators and really any other "indicator mod" I have seen are not allowed as they all provide an unfair advantage to the player using them.