I think, in order for a movie to be truly called an Xmas movie, Xmas needs to be a plot driver of some sort, and not just happen to take place at Xmastime. For example, Die Hard takes place at Xmas but the holiday also drives the plot. John wouldn't be visiting, the company wouldn't be by itself in the building, and there wouldn't be a party where the "terrorists" could get to Takagi without it being Xmas. While you could set the movie during a different time or come up with other reasons for those plot drivers, Xmas makes for a good excuse for all of it, together.
Lethal Weapon, on the other hand, takes place at Xmas but the holiday doesn't drive the plot. Very little of what goes on is *because* it is Xmas; Xmas is just a set design for some scenes. Riggs doesn't become Murtaugh's partner because it is Xmas. The bad guys aren't killing people or bringing in "product" because it is Xmas. Even Xmas-related scenes, like Riggs arresting the crooks in an Xmas tree lot could easily be placed elsewhere without affecting the sequence at all. It could be a used car dealership. It could be a standard nursery. Doesn't matter-- the scene would play out identically without it being Xmas.
Batman Returns is on the edge. Xmas scenes are throughout and some of these scenes drive the plot forward but many others could be set at different times and would play out the same. But the bookend scenes of Batman/Catwoman and Bruce/Selina sharing the "mistletoe can be deadly if you eat/a kiss can be deadlier if you mean" lines would have to be totally changed if you moved the movie to be non-Xmas time. The tree lighting ceremony is a good excuse to get people together, outside, for Shreck and Penguin to frame Batman for murder. Could these scenes be rewritten? Yes. But Xmas makes for a good reason for all of them. It could go either way.
So, in the end, any movie you want to be can be considered an Xmas movie. However, I think if the plot is driven by elements of it being Xmas it qualifies more firmly than if Xmas is just the time that the movie takes place and the fact it is Xmas doesn't drive any of the plot forward.
