A Generic Doomsday Villain is a villain whom is only considered a threat based upon the fact that A) the villain is evil and B) the villain will do evil things because they are evil.
From a designer's standpoint, this villainous archetype fills the void in a storyline when you want the heroes to deal with somebody who is clearly bad news and that failure is not an option.
However, the reason why a Generic Doomsday Villain is registered as part of the Scrappy Index is because an antagonist without any ambitions or motivation usually isn't a well-written one. Occasionally, the villains can lose their motivations, but that is a process of dealing with changes in the story, or the issues arising through character quirks, or more often, new villains show up and take over. But, the important thing is, even with Villain Decay or Motive Decay, those antagonists were, at one time, legitimate threats and both the protagonists and the audience (or at least the latter) were aware of what they were doing. A Generic Doomsday Villain has no such instance because you are never told as to why they want to blow up the Earth, or why they want to enslave a civilization. All you get out of them is that they do these evil deeds simply because the character is evil. As you can guess, redundancy is very common with this trope.
It is possible for a character who starts out as a Generic Doomsday Villain to become a well-defined antagonist with a clear goal or incentive, and it's possible for a villain who shows promise or reasons as to why they wound up as the bad guy to wind up doing very bad things because they are bad. Also, it can be argued that characters who seem to think things through clearly and have lots of screentime devoted to their machinations are merely this trope with good publicity if their goal is not explained. In those cases, such arguments are better reserved for a work's YMMV section as opposed to the main.
Should you wind up seeing a Generic Doomsday Villain, it is okay for a tertiary or secondary antagonist to fill this slot, provided that it merely placates the Story Arc. When the Big Bad itself is this aimless entity, you will run into problems.
But, this doesn't mean that they are all bad. Like all tropes, there is some good that comes out of these characters. Should those that write the media in particular point out the fact that the entity doesn't need motivation to do things that regular Joes can understand, or if they are merely raw power incarnate, then the audience member has a greater chance of accepting that this guy doesn't require a true backstory or mentioning of his desired end-game.
