Can't say I agree with you that self-aware humour was the problem here. The property of Ninjago is inherently fourth-wall breaking due to the fact that the characters are represented as LEGO minifigures in all media. On top of that, the latest season of the TV show has upped the self-awareness and is all the better for it. Self-aware humour and subtle fourth wall breaking can be great when done tastefully. Whether the humour worked or not in this movie I feel is a different question. If you're referring to a particular scene in another movie, I thought it was pretty funny and worked fine with who the characters were, but that's unrelated.
I've only seen the movie recently due to the late home video release in the UK. It did have lots going for it. They assembled a great cast, the animation was beautiful and the soundtrack was great (though I did find the use of the songs 'Heroes' and 'The Power' to be a bit jarring, among others).
But overall the movie was a disappointment. I thought the script was messy and unfocused, most of the characters were paper thin, and it lacked much of the charm that I've come to associate with Ninjago. The whole thing screams 'great big wasted opportunity'. From reading the 'Making of' book it can be seen how interesting this movie might have ended up being, but we've ended up with something painfully uninteresting. The use of all this 'Fellow kids' humour wasn't great either. I cringed way too much while watching this thing.
I have a suspicion that the movie may not have gone through the easiest production. I remember early on that a single director was attached to the project. At some point that changed to 3(!) directors (perhaps that's why the whole thing felt so bland). Combined with the fact that there were 13(!) combined story and screenplay credits, which could mean that there was some difficultly had in actually writing this thing. The early concepts for the way the story went are vastly different to the result we actually got, which isn't indicative of much, but perhaps indicates again that putting the story together wasn't a straightforward process. On top of that, so many clips and shots from the trailers, especially the first didn't make it into the final film, or were changed for no reason. (In fact some jokes worked better in the trailer than in the movie itself!) Maybe the movie was reworked in post production? Might explain the choppy editing in the final release. Again, nothing concrete, just food for thought.
I really wanted this to be good. I'm a big fan of the LEGO movies and of Ninjago so really wanted this to succeed. It's a shame then. At least the new Ninjago city design and DB are being used in the show so they don't have the shadow of this ultimately sub-par product lingering over them.