Okay, time for another edition of Tim Thomason's always hilarious classic joke breakdown:
This is the first line, what you quoted out of context. It is a bog-standard criticism of the movie, itself offering nothing of value and no real critique, just stating that it is "really bad" and then a suggestion not to view it. Taken by itself, this could be seen as a joke on criticisms that don't go into detail, but I will admit that that would be too subtle for most, so that's why a second line exists to clearly show that it was a joke.
The second line (or punchline, if you will):
There we have it. A punch to the gut that clearly sent most people doubling over in stitches, or something. See, the author (me) is admitting to not having seen a movie that he just critiqued as "a really bad movie". How can he criticize a movie he hasn't seen? He can't. That's the joke. He's pretending (or rather, just saying) a movie is really bad based on, perhaps, Internet hate or pop culture belief without any true opinion of his own. The joke is a critique, in and of itself, of the type of people who offer uneducated opinions or beliefs just because they are popular on the internet, with no addition or correction of their own. I.e. completely worthless commentary.
The "Unrelated" bit adds an extra layer to the joke, already rich in detail, that makes the author (me) seem completely oblivious to the fact that his not having viewed the film while also somehow knowing it is "really bad" are diametrically opposed to each other. It's a way of reinforcing the fact that the author (me) is an idiot and shouldn't be trusted with regards to his opinion.