And if not thwarting, at least doing something about it, even if it is just posting on the internet or preaching in the streets.
Indeed I think the standard of effectiveness centers around exposure of the nefarious activity, not alleviation. In fact the sentiment, "There's nothing we can do to stop them!" factors into the mythos. But to belabor my previous point, the presumption is that the conspirators want their actions to be kept secret. Exposing them to the public is perceived to have a deleterious effect.
It's interesting that there aren't any popular "good" conspiracy theories, ...
Theories, no. But there are plenty of beneficial conspiracies in fact. Go look at the donations page of any charitable, artistic, or philanthropic organization and you'll find a substantial number of anonymous donors. There is indeed a conspiracy on the part of the wealthy to enrich the lives of their fellow beings.
But your point isn't lost: the effect of conspiracy theories to create a devil is what drives the eventual "victory." That's pointless if the conspiracy isn't aimed at oppression.
I guess some of the UFO fanatics believe that aliens are giving governments technology to help us and lead us into some intergalactic brotherhood of peace-loving sentient beings or whatever...
Emphasis on the "whatever," since most of these morph into theories that governments are using secret alien technology to maintain their oppression. Plus there is the sheer ideological misgiving about keeping the aliens secret. UFO proponents bark incessantly for "disclosure." They believe they've found a secret and they want the people they accuse of keeping it simply to come clean about it.
I agree that for UFO believers the alleged aliens fill the Mentor or Protector archetype, and so act in the same capacity as deity or angels in a more traditional religious mythos. If you look at how the characterization of extraterrestrials evolves over time, the aliens always seem more adept at whatever social or moral difficulty we happen to face at the time.
One prominent researcher dealing with alleged contactees noted that the aliens -- who are presumed to be so much more technically advanced than we -- are unable to tell their Earthling contactees any information about their technology breakthroughs, but are oh-so-full of helpful moral and social advice.