There is a lot that goes into making an app like this, and a lot of what you would need is not publicly available. The only public API that Newgrounds has is user authentication. I don't know if they have any other API's, but they are not listed. Trying to get this kind of data via the web is not viable.
There's also the question of apps versus mobile web. This type of website would likely benefit better from developing a mobile version of the site, rather than forcing users to download an app. However, they have tried this in the past with the PSP version of the site - which likely didn't have enough traffic to make it important enough to continue.
Newgrounds is a site that people visit on computers, and less on smartphones and tablets, especially since the majority of the site is based in Flash. I'll agree that there are some fun things to think about on this topic, but I'm just not sure it would be used widely enough to be worth putting the effort into.