While in theory Morathi should spread corruption in the game it simply doesn't work well.
Firstly the fact that the spread of corruption makes it extrewely hard to do diplomacy with anyone. The people who should be your allies won't even trade with you and the chaos factions see you as just another target. So you can't make friends...at least not easily and everyone hates you. The whole of the Helf faction just chain invite each other to go to war with you, that seems inevitable. I can't even get a trade deal with Naggaroth half the time. The other issue is most of the smaller sub factions you could have dealt with before aren't surviving long enough to be relevant.
I personally think its far too overt for Morathi, she never likes to show her hand and always plays on the role of being loyal to Khaine even though she isn't.
So far as i remember from the old world she and her son were for the most part a team and perhaps Melekith didn't trust his mother much he usually worked with her.
If she is to spread corruption then should chaos really be attacking her like everyone else? If she is going to be an agent of chaos she should get some benefit for it surely? Its just all negatives with the corruption spread. Morathi maybe corrupt but shes not an idiot. she certainly wouldn't want everyone to turn on her.
TLDR - It makes her too isolated as a faction as getting any long term allies is unlikely where as the amount of factions that want to destroy her is rather large. All the time there is no real benefits coming from it at all.