This episode was weak compared to many of the others. It would have been believable if Mumm-Rana's powers were more equaled to Mumm-Ra's. Her defeat and falling under his spell happened way too fast.
It also seems highly unbelievable that the two of them would be eternal beings living on the same planet, yet she at least had no prior knowledge of him when apparantly he had wreaked havoc on the planet prior to the ThunderCats arrival. It would have been believable if the two of them had a history together of fighting against one another.
This episode could have greatly improved if these two points had been given more consideration.