The built in internet browser is actually not that bad, with the 3DS' slow CPU I don't think there would be much of an improvement with a homebrew one. It would have more RAM accessible which might help a little on massive web pages, but I don't think it would be worth the effort porting an existing browser and certainly not creating a new one just for the 3DS, especially not since modern browsers aren't designed to work well on low end systems like that so it would have to either be a port of an older browser or one designed for similar systems, or an entirely new browser.