I concur. Mumakill are not really what they should be right now, in my opinion. Haradrim Archers or Spear Throwers would be a nice addition and give them some light form of defense against Pikemen.
I see a couple of ways to do it, but I think the most reasonable way would be to give the Mumakill archers either by default or by influencing Tribute Camps with Sauron.
Perhaps, if we were to give the Return of the King some credit, it might be a nice feature to have it start with archers and perhaps they would be upgraded to Spear Throwers when the Tribute Camp is influenced.
Nonetheless, the fewer Harad and Easterling units we give Mordor, the more possibilities we have to play with for a (very possible) future Haradrim/Easterling/Khand/Evil Men faction
.