This is why some of you shouldn't be answering questions like these.
NO, those screenshots usually aren't of the highest quality, "highest quality" being a rather subjective term. Depending on the tools used, the screenshots are usually taken directly from the frame buffer. While this does yield pretty "crisp" screens, it doesn't give you a very good idea of what you're going to see on your screen. TV screens do some natural filtering and smoothing that isn't evident in those screens.
In fact, one of the companies I used to work with would run all screens through a Photoshop filter that would add some of this smoothing so that the screenshots would look more like what people would expect. As far as I can tell, Nintendo doesn't do this.