With regards to anime usually being massively overacted in Japanese as well, that's clearly true. But at the same time, it's definitely an intentional choice, and I believe in many ways necessary. If you go and listen to an audio drama on the radio or something, Japanese or English, you'll find they all have a similar level of overacting; it's because, unlike proper TV and film, the actors can't make use of body language, facial expressions, etc - they're restricted to using nothing but their voice. Therefore, in many situations, the only way to make sure they can convey the emotions or feelings they want to is to hugely over-emphasise them, hence the overacting. And with a lot of Japanese comedy, it's traditional, in a way, too, which is also a factor in some anime.
Of course, there are also anime where the actors act normally, without the overacting you usually see, and it worked very well (examples include Whisper of the Heart, 5cm Per Second, and I *think* The Sky Crawlers, though I can't remember that one exactly). Of course, these are all films which, to an extent, convey deep emotions in other ways less commonly seen in anime, and which have much more detailed animation in general, which also helps. So whether or not it can work in all cases I have no idea, but overacting is certainly the commonly accepted best practice in mediums where only the voice can really be used, in both Japanese and English.
Anyway, I don't think most people find the overacting to be the problem with dubs. While poor acting on the part of the cast can often be a factor, it's more due to poor casting (so characters end up with voices that do not fit with the original at all), dubs conveying meanings that are slightly different to - or even sometimes totally at odds with - the original, and (in my case at least) those fucking horrible accents. Actually, for me, it's almost entirely the accents which put me off, they really grate. They're really difficult to listen to.
But personally, I mainly watch anime for the Japanese immersion and listening practice anyway, so dubs are basically irrelevant to me.
