Okay, so I'm about to totally rail on Batman Ninja, but before I do that, I wanted to open it up with the following disclaimer: I thought multiple times throughout watching this "I wonder if this could be the sort of thing that's fun if you watched it in a group setting?". Personally, I watched it by myself, on a bad day, bedridden, feeling very, very unwell. And that's the experience I'm reviewing.
Movies don't have to be grounded to be good. They don't have to be be standardised to be good. They don't even have to make sense to be good. Sometimes a movie is just silly fun and everything else is moot. But most of the time, the better a movie is (ie. The more you as the individual viewer are appreciating your experience) the more nonsense you are willing to forgive. It's easier to suspend disbelief, even a great big fat disbelief, if everything else in the movie is working. If not everything is working, but you're only called upon to accept a small amount of ridiculousness, then that will generally fly. But in some movies... In some movies, there is absolutely nothing that is working, and yet those same movies will ask you for the biggest, fattest suspension of disbelief of them all. Batman Ninja, is one of those movies.
I could probably pick any one aspect I disliked about Batman Ninja and write a full length review based on that single issue, but I generally try to keep these things pretty short, so rather than get into the plot, the deviation from source, the character choices, the dubbing, the animation style, the sexism, the science, the lapses in logic, the design choices, the dialogue, the nonsense, the inconsistencies within its own defined setting, the fact I was so bored at one point I literally fell asleep, or any number of the problems I've either blocked out or to list would require spoilers, I'll skip over all of that and simply say: This is the worst Batman movie I've ever seen.
Final rating:★ - Of no value. Avoid at all costs.