I know this is kind of silly to even consider but hear me out. First off think about it. Until the Mutants flew by the black pyramid Mumm-Ra didn't really do anything when it came to the Thundercat's ship crashing. Also if you consider how inactive he seemed to be before the beginning of the series it seemed that he prefered to just snooze in his sarcophagus or watch TV in is cauldron's water. Yes the other beings on the planet knew about him and his pyramid but they honestly didn't seem that concerned about him threatening anybody. Maybe by then he'd basically dominated the planet and felt no need to be active thus basically letting the inhabitants do whatever they wanted. Also consider the fact he had taken out a LOT of other villains, like the lunataks and Amuro(?). There's also how he reacted to the thundercats when they decided to put up the Tower of Omens. He seemed very upset that random aliens were building architecture on the planet. HIS planet. Consider this. Would you want some alien from another planet coming to yours and deciding your planet belongs to them and if you do anything about it they will beat you back over and over, ignoring the fact that you have a bigger claim on the planet than they do? If you think about it it's like it's just one being defending the planet from a pack of entitled alien invaders. I know it's rather silly and I'm sure my point of view of this is very likely wrong but I just am dorky enough to consider things from his point of view. That being said I don't hate the Thundercats at all.