The documentation CLEARLY indicates that the HDMI output has ONLY been validated at 1920x1080 “1080P” resolution.
Your 800x480 monitor probably has built-in scaling, but yes, it will have fuzzy output and possibly flicker, since 1920x1080 does NOT map by whole-number multiple to 800x480 resolution. What I mean by that, is if the target resolution was exactly HALF of the source resolution (i.e. 960x540), then it would be able to scale by simply cutting out half of the pixels in each direction. In your case though, you are scaling by a ratio of 0.4167, which means that it has to GUESS at the value of most pixels. This guessing can be done fairly well, but at computational cost. It can’t really be done well with the board’s integrated controller.
Now, there is no documentation on this, but there is a way to get an output resolution of 1280x720, but with the stock software, no sound. There is a way to get sound back at 1280x720, as well as achieve a resolution of 1280x800 if required, but both require custom kernel modifications.
If you want resolution to automatically match your monitor, currently, you are required to use the Ubuntu images. Android currently won’t do it.