While playing modded 1.12.2, I noticed that Mob Masher does no damage, after checking out I found the cause to be NoDamI mod. Not sure if this is something that can be fixed or if the way the mod works would not allow it.

Minecraft Version: 1.12.2
Forge Version: 14.23.5.2859
Mob Grinding Utils Version: 0.3.13
NoDamI version: 1.12.2-1.3.1

Fairly unlikely to be able to fix, the mod probably fiddles with the whole combat system, and its 1.12, which i don't even have a functioning dev environment for anymore, unfortunately.