Back in 2014, I thought The LEGO Movie was great, and I thought LEGO Batman was one of the best things about it, so I really expected to like this more. But…
It's a lighthearted look at Batman's dysfunctional go-it-alone-ism. On one side, he refuses to accept the help of would-be allies. On the other, he avoids acknowledging the necessity of his relationship with his perennial villain adversaries, particularly the Joker.
That neurosis is tested when Joker is sent to the Phantom Zone, where a lot of bad guys get dispatched at the end of their movies: there's Sauron, Voldemort, Godzilla, Kong, even Jaws (the shark, not Richard Kiel). Joker promptly dumps his old allies (Harley Quinn, Bane, Riddler, …) and uses the new ones to unleash a fiendish plot against Gotham. It turns out that Batman needs the help of his own crew: the accidentally-acquired Robin, Alfred, and Barbara Gordon.
All this is accompanied by a lot of jokes, both inside jokes and outside jokes, and they're pretty clever, and thank goodness for subtitles, because I would have missed a lot of them otherwise. But the teamwork-is-good moral is laid on with a heavy hand, and that doesn't help the movie.