Monologues and voiceover can be done, it's there a lot in noir type stuff. But I think the way to go with that would just be as an opener to the story, a little "My name is.... ______ 30 seconds or so of monologue" at the start, kind of like the first Raimi Spiderman.
I've always thought maybe the best way to do Animorphs would be segmenting it a little, the "big event books" with some significant development and reasonably large scale would be movies, with an animated miniseries sort of bridging them. But then you'd get a situation of people having to view the show between movies, and so many people wouldn't bother.
It's more just a combination of the special effects (live-action living things aren't quite there yet with CG, Avatar's close but even that's aliens, made up things where we're not intimately familiar with what they actually should look like) and more importantly content with Animorphs. You've basically got two options: sanitize it so it's acceptable, or don't make it. You don't get to do this the way the books were, not in reality.