I honestly don't quite know how this one will play out, only insomuch that Ninjago has been a TV cartoon for some time now. I think that when Lego Batman the movie comes out, film goers will know that there's a clear separation between the constant 2 a year flow of Lego Batman/DC Comics DTV's and Lego Movie Batman. That's the deal with that Lego video game, and I think most that followed it know that DC Comics Lego Batman is a different continuity and character, and also predated the movie for a few years. Plus, Lego Movie Batman was the scene stealer for that film, so no doubt his spinoff is going to be wall to wall wackiness. Plus it's Batman and I'm confident a fun version of the character will be the perfect antidote for the overly dark Batman VS Superman film.
But a movie based on a Toy Line that's also a TV show seems to be the trick. Removing the obvious question of will it be a Spongebob/Rugrats/Simpsons level of success or a Hey Arnold/Ratchet and Clank (which is technically not a TV show, but the movie felt like the movie to one) flop, there's going to be a major disconnect. This will be a Ninjago movie that's stylistically similar to the Lego movie, rather than the TV series. And it feels it could go either way. I don't think a direct film adaption of the TV series would excite any more casual types, at the same time, I could see kids who watch the series be confused by the characters and themes being different.