#monkey foothills

It's heavily implied that the Monkey Minions are greatly made up of those from the PPG Movie, which Mojo had taken from the Townsville Zoo and injected with Chemical X, and/or their descendants. (A big hint to this is the naming scheme of many of the Monkey Minions fitting those in the film.) Due to their heightened intelligence, many of them couldn't fit in at the zoo anymore even if some of them might've wanted to go back to that life. There were also those who, while having failed to overthrow Mojo in the film, nevertheless still didn't want to serve him--as well as others like Mopey Popo in "Prime Mates" who would choose to turn good. As a result, a good chunk of them moved to Monkey Mountain and its surrounding foothills to create a society of their own. This society would only grow in later years as more simians came to know of it. Despite how it was able to thrive in harsh conditions, it being so isolated off from the world meant that it lacked many resources--specifically with technology. When Mojo took it over, there was some resistance; however, in the end, most of the monkeys there gave into Mojo eventually due to their previous history, shared superiority complex against humans, and the desire for the outside resources that Mojo could provide. Not everyone who lived there was happy to be back under his thumb: While there were some strong followers, to many, it was just a deal that had to be made.

There are a few key reasons why Mojo took over the area other than that it allowed him to get some much-needed man--or rather monkey--power. It being so isolated in the depths of the wilderness meant that he could conduct many of his plans there in secrecy. It being so far from Townsville itself, not even the Powerpuff Girls would give much attention to what he was up to there. He had already figured out how to build a highly advanced base of operations in a volcanic setting thanks to the construction of his own home in Townsville. Furthermore, the resistance he did face there was fairly minimal thanks to, again, his past dominance over the other monkeys and bargaining power--albeit, he has no doubt that some of the other monkeys can still try to overthrow him someday. This knowledge is hinted at in the mission "Monkey Shines," where he tries to use Chemical X and a Current Diverter to create more minions for himself.

As much as it was also due to his own ego, Mojo had many large statues of himself built in the Monkey Foothills to make his presence felt there. Their reorganization into a large army, their newfound resources, and any growth seen in the area was thanks to him and the statues were there to remind the other monkeys of just that... as well as just who they were dealing with if they betrayed him again. We see the ruins of these statues scattered in the foothills in the game.

The reason for the ruins and why there's no developed society left standing there as of the events of the game is thanks to the war and the big role Mojo had at the start of it. In the promotional manga, FusionFall: Worlds Collide, Mojo and his army were the first to learn of the invasion and fight Fuse. Even after their sounding defeat, Fuse was still wary of the group for how quickly they responded to his arrival to Earth's system. Monkey Mountain was heavily targeted as a result, suffering some of the worst damages at the start of the war. Beyond that, once the area's population had to flee in response, there was little left to maintain what managed to survive that attack: Failsafes and defense mechanisms such down, and the locations own volcanic activity destroyed much of what remained. Now, there's not much left outside of camps in the outskirts of the foothills.

For both Mojo and the monkeys that made their home there, beating Fuse is a matter of pride and revenge. It's also one of the biggest reasons why they joined hands with the Fusion Fighters so willingly. Many of them still keep their feelings of superiority toward humans and the rest of Earth's creatures, but the fight with Fuse is personal: So, yes, they'll put up with the bad monkey jokes and ferry people around if it means eventually giving back the damage they received tenfold. As covered with the Monkey Minions themselves though, this alliance would end up creating more positive relations between said groups in the long-term. It's unknown as of yet how those split feelings might affect life on Monkey Mountain once the war is over.

Some Monkey Minions continue to monitor/explore the area despite how much of it is in ruin. This is mostly to take back what land they can and ensure that the infection surrounding the mountain doesn't spread further. Those aware of Fusion Mojo Jojo's activity there feel some extra salt in their wounds and will make a point to target him and his lair in-particular.