What should a small bump on the rim of the upper eyelid be?

Bumps. These are usually plugged glands. But they can be many other things. Best to have someone look especially if growing or painful and scabbin.
Stye. Occluded meibomian (oil) glands along the edges of the lid can get stopped up and cause a bump or swelling along the lid margin. Other causes can be papilloma (wart), skin tumors, or even irregular skin growth. See an ophthalmologist to see if it is prudent to remove it.