Lady Gaga and Joaquin Phoenix don't seem to be enough on their own to make the 'Joker' sequel a hit ... and the early returns are anything but promising.

"Joker: Folie à Deux" is stumbling out of the gate ... making only $20 million at the domestic box office Friday, and that's not even the worst part.

The Todd Phillips-led movie is clearly bombing with moviegoers ... earning a "D" grade on CinemaScore. It's a record, albeit a bad one ... the lowest CinemaScore grade ever given to a comic book movie. Yes -- lower than "Madame Web."

Rotten Tomatoes isn't laughing with 'Joker 2' either ... giving the movie a 32% critic score, which is just a tick above the film's 31% audience score. Splat.

Joaquin won an Oscar for the first go-round, but this flick combines a love story with a musical ... it's not going over well, and folks are ripping it to shreds online.

Projections for the opening weekend box office are sitting below $50 million ... worth noting because the budget was reportedly $200 million, a huge increase from the $65 million needed to make the first film.