I'm not decided on the memes.
I see pieces like this to be similar to a collage in a modern art display. It's as if a person were to cut out parts of a newspaper and paste them together to make a new story.
In the end the artist has made a new creation that is very different from the first.
But still it would be a good courtesy to source the original newspaper used in the collage in the description of the piece.
The problem is not many can go through the trouble of tracing a MEME or gif to its origins. They are evolving from hand to hand until the original source is difficult to determine. I think @holovision would know better than I about when a meme is original content and when it is considered plagiarized material.