Improper Monsters are GMod Monsters that are badly made, have no feasible flaws or weaknesses without the powers to justify the absence, can be summed up as "bad copy of an existing monster," or attempt to meet/go beyond the Killer Cake Limit. They can probably be improved but it will take work. Lots of it. That or can be endlessly mocked to one's heart's content, as long as doing so is done on the talk page or, as in the case of Death Demo, authorized by an admin.
If you are planning to make a monster, these can be good examples of bad monsters. Avoid making your monster similiar to these, and it will probably be good.