I recently uploaded two posts I'd describe as a "collage", post #6388451 and post #6392827. However, upon inspection of the collage tag it seems to be just multiple_views 2.0.
A quick scroll through its posts does reveal some collages (post #6375655, post #6379170 etc.), but they're drowned out by a large amount of plain old multiple_views posts (post #6305513, post #6288150, post #6381295, post #6381049, post #6361362 and many more). There's also the occasional ??? post (post #4258109, post #5572780) where I have no idea why they've been tagged at all.
collage is also used not only in addition to, but in place of multiple_views. The post count for collage -multiple_views chartags:1 is 726, and these posts in particular are almost all just multiple_views. There are very few actual collages in this search.
A tag for collages is a good idea in theory, but the current collage tag needs some major cleanup if we want to keep it around. Considering the state it's in, however, it might be easier to just deprecate it and start again with a new tag.