She kept shut for her daughter's sake, because she knows CC loves Manav and wants to marry him ... and Manav was ready to leave his mother for Chanchan too. So she has some faith in Manav .
The day CC says no for the rishta, or Manav stops supporting her, CC's mom will not keep quiet. If she insults UB or says anything, and the rishta goes ahead, CC would have a worse time in the house.
Women are pragmatic and think of these things. And that's the grey area this show is bringing up ... should she have kept quiet, or should she have said something, knowing it might rebound on her daughter later?
The show, I can see is trying to play with the difficulties of the Indian society , especially regarding women. I get it.
But, I cannot take Uma Ben's insults. She's leaving no stone unturned in insulting CC's family, in every way.