Taking into consideration that if Gaia had to clean up every single mess humanity made, they would become weak and too compliant: It's one thing to teach kids to try and fix the mess humans have made of Earth so that their future isn't a disaster, but when the world has got people like Dr. Blight, Verminous Skumm, Duke Nukem, and Zarm screwing things up for their own personal agenda or just for the hell of it, it's only natural she'd react by giving them the tools necessary to deal with threats that are too big to be handled by just recycling and conserving energy.
Lots of people (mostly harping the Nostalgia Critic and Linkara) like to say that the Eco Villains were just polluting for the hell of it, but close inspection of the series shows that most had an insidious, not-entirely-insane reason for doing what they do:
Hoggish Greedly and Looten Plunder were, quite literally, Gorgon Gecko Expy 1 and 2...all they cared about was their business interests (Hoggish cared about his son, but that's the sole exception). They did what they did not out of malice but bottomless greed and lust for cold hard cash.
One episode even shows Looten Plunder as more of a Well-Intentioned Extremist - he brings jobs and money to a poor indian reservation. A noble goal and all, but he doesn't do so in an environmentally friendly way and faces disastrous consequences. He even agrees with the planeteers when they take advantage of what they have.
Likewise, Sly Sludge was just a lazy asshole who wanted to get rich quick, and while a mad scientist and fairly brilliant, just like the others were, once again his bottomless greed and avarice drove his every thought and action. With the exception of Greedly they were all just sociopaths.
Dr. Blight was simply insane, the result of (no shit) botched plastic surgery —she did something to make herself look like she does, but complications destroyed part of her face and drove her insane — and her actions can be simply explained as "she doesn't know any better".
Duke Nukem and Verminus Scum both have the same purpose and, in fact, worked together in the past: they want to wipe out mankind as it exists and replace it with their own kind, and the fact they openly work together on more than one occasion shows they probably agree as to what "their own kind" is — mutants deviated/evolved from normal humanity by exposure to pollution. Duke Nukem was vaguely sympathetic in that he couldn't live without radiation any more and in many ways he HAD to do what he did to survive.
Only Zarm, the spirit of war and hatred, wanted to destroy, despoil and kill just for the hell of it. Which makes sense—that's why he was exiled in the first place, because he wanted to conquer instead of rule peacefully.
Linka's only stated to come from "The Soviet Union" and later on "Eastern Europe". It's highly possible she might be Ukranian or Belarusian, rather than Russian.
The pilot gives further credence to this since when Wheeler remarks "Love your accent, babe. You Russian?", Linka's response is a disdainful "Soviet. Please go avay.", implying that she is NOT Russian and finds the assumption insulting.
After some thinking I noticed something about Wheeler. When the Planeteers all get their rings, Kwame, Gi and Linka are just doing things they normally do and Mai Ti rescues an animal. What's Wheeler doing? He's stopping a thug. In addition he gets control of the one ring that's hardest to use without maiming or killing someone and tends to manifest in a tight laser beam. While the other rings might be able to do more damage, Wheeler gets the precision weapon. He's also less environmentally aware than the others. What does this mean? Gaia didn't need another person who knew the best way to save trees, she wanted someone who could fight and gave him the ability to end a threat if needed, then and there.
We've already seen that the Heart Ring can be potentially the deadliest of the five Rings Gaia gave to the Planeteers. So why did she hand it to a kinda little boy who'd never use it to its full potential because of his kindness? She did it because Ma-Ti wouldn't abuse said powers of the Heart Ring — which, if both the series and the comics are believed, can potentially include terrific skills like More Than Mind Control, Brain Washing, command of wild life, knocking people out, etc. Confirmed with The Bad Future and It's a Wonderful Life episodes, which show what would Ma-Ti do if he used the Ring for his own benefit....