This statement is a bit vague. What clearly makes it worse?
You can achieve a much higher graphics resolution on an emulator, as the original N64 was designed to display graphics on relatively-low-resolution televisions. You can also enhance certain games with texture packs. But emulators are designed to run at the same speed as the device they emulate, so the framerate (which is hard-coded into the games AFAIK) should be the same as on the original console. (Like Agozer said, NTSC games may appear to run slightly smoother than PAL, which isn't really unique to emulators.)