the games emulated by mame cover a very wide range of resolutions.
For non-3d games, running them at anything other than their native resolution must either be handled by some kind of filtering, or dealing with visual artifacts.
3D games in mame will go through the same sort of scaling issues, but those games which are also emulated by some other emulator that renders 3D using the computer's video card may be able to render the game at an arbitrarily higher resolution, rendering the polygons with more detail than the original hardware could (though textures will usually not be improved any).
You can see a list of resolutions and the games that use them at MAWS:
http://maws.mameworld.info/maws/use the "deluxe search" at the top.