Here are 2 examples, 1 from a 2D game, and 1 from a 3D game:
On the left, it's a Screenshot, taken from a DS Phat.
On the right, it's a Screenshot, taken from a 3DS.
On the left, it's a Screenshot, taken from a DS Phat.
On the right, it's a Screenshot, taken from a 3DS.
Maybe you could see some blur on the 3DS Screenshots, but that's because DS games are played on native 3DS resolution.
To play it on DS's resolution, just hold Select or Start, while booting a DS game.
This will make the game very small, but at least it won't be blurry anymore.
Apart from the DS and DSi Modes, 3DS Mode Games and Apps are worth it.
You can't play any 3DS game on a DS or DSi, but you can vice versa.
In these days, there are more new good 3DS games coming, than new good DS games.