I would venture to say yes, although I don't necessarily think it's entirely to do with the importance of inner monologue to the original material. That being said, it definitely plays a role - since so much of the book is devoted to the characters' thoughts and feelings, it leaves a lot to be said for the actual action that takes place, which feels lacking at times. I've had this conversation with my mother a few times and we've agreed that as the films progress, they get harder and harder to follow if you haven't read the books due to what is left out and what cannot be portrayed well in film, like internal monologue.
But I think the other reason Deathly Hallows feels weak at times is because it's a set-up for part 2. It suffers from what I like to call "middle-book syndrome," where all the action leads up to a conclusion that isn't contained within the film/book itself, thereby leaving it feeling weak and incomplete in and of itself. Deathly Hallows is weak because it's missing its second half; I think if you view the two films together they will feel stronger.
So yes, but for other reasons as well. Which is not to say that I don't like Deathly Hallows - I think the Yates films are the best of the series.