For 3D games, you can use a cheat code to render a wider field of view (models squashed horizontally - taking up fewer pixels so being lower quality - into the DS's 256x192 resolution, then stretched back to the correct ratio by the file created by the patcher).
For games that don't have a field of view code identified, and any 2D games, yes, the picture will be stretched and it will look wrong (c.f. the GBA's 1.5x widescreen mode for GB[C] games).
And it won't look as pretty as the regular aspect-ratio scaling modes, because expanding 256 pixels to 384 required deactivating the improved scaling for the x-axis.
