Dumbo being mysteriously silent made little sense to me and turns me off from the movie after rewatching it. I agree he's a sympathetic mute lead, but it still makes the film not quite as good as it might could have been.
Most people view the Ringmaster as the main villain but he never really did anything "evil" outside of having Mrs. Jumbo locked up.
I excused it simply because I acknowledged that Dumbo was a baby, and babies shouldn't be able to speak. Even Bambi couldn't speak for a little while. Sure he did eventually, but the amount of time that passed with Bambi was much longer than Dumbo. Dumbo just never grew to be old enough to speak I guess. If he spoke throughout the film I don't think I would like it as much.
The Ringmaster is is seen as the main villain? Really? I don't remember him being evil at all. Locking up Dumbo's mother was just something he thought he had to do after she started a commotion. I don't think there was any malicious intent behind it. And yeah, the comparison with Cinderella's sister is a pretty good one; I can see the similarities.