As someone who followed the trajectory VERY loosely, it seems like:
Dragon Ball Super (Goku reaching Ultra Instinct) > A clip from a straight-to-DVD of Shaggy being uncharacteristically tough, replacing the scene with Ultra Instinct effects and music> "Shaggy can beat Jiren and Goku" jokes > *silence* > A quick clip of Shaggy being buff from one of the live-action films > Reddit/Twitter starting the idea over how Shaggy has the power of a god > The guy who played live-action Shaggy endorsing the meme > Meme lords trying to get Shaggy into MK11 because Warner Bros. > ??? > This
tl;dr - Memes are dumb and the internet was a mistake.