What do you call a fake abbreviation? Like saying that CNN stands for Crap News Network instead of Cable News Network. What do you call it?
An acronym is an initialism that forms a pronounceable word, like SCUBA or RADAR or NASA. If someone develops an initialism for the express purpose of spelling a particular word, for example the USA PATRIOT Act,1 that is called a backronym (backwards acronym).
"CNN" is only an abbreviation, not an acronym, because "CNN" is not a pronounceable word. But you might be able to stretch the definition and say that "Crap News Network" is a backronym for CNN.
1Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act