“Monster-of-the-week” is a term used to characterise television shows that rely on the regular appearance of some sort of creature to drive forward the action in each episode. The phrase is usually disparaging, as when applied to the latter half of the series Voyage to the Bottom of the Sea, or to Lost in Space, but is more neutral when used to describe shows like The Outer Limits.
I think that you have to be careful about throwing out labels to describe Lost in Space, though. Monsters made very numerous appearances throughout the series, but they are not all of a type. There are a number of distinct purposes for the appearance of one, and I would like to list a few.
First of all is the category “danger.” This type of monster is the usual reason for decrying a show as being monster-of-the-week. Surprising as it might be to some, there are not that many occasions in Lost in Space when a creature appears in a single scene for no other purpose than to menace a character and then be destroyed or driven off. Two examples that do come to mind, though, are the giant spider in “The Forbidden World” and the ruby eating creature in the teaser at the beginning of “Revolt of the Androids.” In neither case is the creature in any way integral to the plot; rather, it serves as a device to add a momentary element of danger to the story and to underline the fact that the Robinsons are living in a hostile environment.
A second category might be called “mistaken identity.” These monsters appear menacing, but turn up in order to perform a task. An example of this is the shaggy, one-eyed monster in “The Prisoners of Space.” This same monster appeared in "The Magic Mirror" where it was in fact menacing. Its task in "Prisoners" is to deliver the communications device to the Robinsons that will inform them of the charges to which they must answer in the Galactic Tribunal—it's a sort of space bailiff. The same creature later shows up in the background of the courtroom scenes; it must be assumed that it is actually a sentient alien, not an alien animal. In the same vein, Bolix’s “bloodhounds” in “All that Glitters,” while clearly not sentient, and certainly menacing, are nevertheless animals under the control of a master. The same can be said, more or less, for the cosmic monster in “Space Circus.”
As a third category, there are monsters that are integral to the plot of an episode or to a subplot within the episode. Quano’s beast in the cave in “The Challenge” is an example of this. Other examples are Uncle Angus in “The Astral Traveller” and the bush creature in “The Raft.”
Finally, something should be said about recycled monsters. Recycled monsters may even be partly responsible for the “monster-of-the-week” label some television shows earn. Recycled monsters are creatures that appear in a number of episodes (and in Irwin Allen’s case, a number of series) as a cost-saving measure. Chronos’ assistant in “The Time Merchant” appeared earlier, caged, in “Hunter’s Moon” as well as in the Galactic Tribunal in “The Prisoners of Space” and in the cave scene of "The Anti-Matter Man." The giant spider from “The Forbidden World” was also in the courtroom of “The Prisoners of Space,” as was the bush monster from “The Raft.” I also wonder if Uncle Angus from “The Astral Traveller” wasn’t a redress of the same make-up. The alien who appeared in the final act of “A Change of Space” to reclaim his stolen property had appeared in Voyage to the Bottom of the Sea. Although cost-effective from the standpoint of producers (especially when they are used in multiple series), it is undeniable that their use to pad out crowd scenes leads to continuity issues and leaves an indelible impression on viewers that they have already “seen that one somewhere before.”