First of all, everyone keeps saying hold start and select, but you only need to hold select, not both.
Anyway, some games look good, others don't.
Games with clearly defined pixel art (Pokémon games, Chrono Trigger, Phoenix Wright…) look bad. Hold select while you start them and play at the original resolution. They're still playable and in many cases will look fine.
Games without clearly defined pixel art (Elite Beat Agents, Mario & Luigi series, Kirby: Canvas Curse) look just as great as on the DS.
Some games don't look better either way, or you need it larger. For example, Trauma Center would be waaaay harder with the screen so small.