the game does not delve deep in it yet, the only thing you get is nergal's actions and erevi's explanations about the supposed beef with qetesh priestesses so far, only time will tell what tinker will come up with.
you should read the quote chain, qetesh is aware that mc found a way to bypass the anti-baby potions they used to drink and stopped zyra from lifus deleetus the mc once she found out about the enhanced fertility potions, their priestess being able to concieve children is not a problem for qetesh right now nor does undermine/weaken her in any way, not even forgetting to do mass seem to do anything, besides the girl that developed enhanced fertility was a nergal cultist according to her diary and the war prevented her from being discovered as the head priestess was sussy due to pregnancies happening, this is why i said "...ignoring the fact that qetesh is fine with the MC..."
if the MC was a problem/nergal underminer for qetesh she would have not stopped zyra in that ghost babies scene.