Never settle for a simplistic single variable explanation. The Flash flopped for a dozen reasons, including some out of Muschietti's control (studio shake-up, Snyderverse implosion, dead-end narrative, etc.), but to say that people just "don't like the Flash" is absurd.

People forget this now, but Iron Man was far from a popular character. Even a lot of serious comic book fans didn't know much about the Guardians of the Galaxy. But good writing and directing and a studio that wasn't in complete turmoil gave us all characters to care about and root for. 

And also, keep in mind that the CW had an incredibly popular series about The Flash starring Grant Gustin that ran for 9 seasons!

The Flash is a top-tier DC character and one of the more fun ones in general, because he's funny and earnest and actually wants to be a hero. 

But DCEU's "The Flash" was a bloated mess that was sort of trying to be Flashpoint Paradox, but also 10 other things, while bringing in 'member-berries Batman, alt-universe Supergirl, an insanely annoying alt-Barry, and the whole thing was just super disjointed and overly long.

And that's all before we say a word about the atrocious VFX.

Nah, man... You just made a bad film.