I've only played one DS game on my 3DS, Pokemon Black 2, and I had absolutely no problem with the way it looked. It looked almost identical to what Black 1 looked like on my DSi, and I don't know what's up with people complaining about a shrunken screen. It takes up the size of a regular DS screen on my 3DS. And all this is coming from someone who played all the other DS Pokemon games on the original DS/DSi.
Although, Pokemon games look awful on a 3DS XL. --- Good gameplay can easily salvage a bad story. Good story can never save bad gameplay.
It's fine, nothing to bust a nut over. Worst case, get an XL and play in native resolution. Problem solved. --- Now Playing: Monster Hunter Freedom Unite, Terraria, The Ur-Quan Masters, Nexus: The Jupiter Incident.