An electrician put an electrical outlet box in our covered porch ceiling. We want to hang a light fixture (one with a cord and plug over a dining table). I will be installing the outlet myself. Do I need a GFCI outlet or just a 15 amp regular outlet? The wiring is in the attic above the ceiling. The porch has sliding glass windows, the ceiling cannot get wet from rain, but there is normal humidity when the windows are open.
It sounds like you described a damp area, like a bathroom. Bathroom lighting does not require GFCI. But...
First, you are never required to use a GFCI receptacle. You can use a plain receptacle, and if GFCI protection is even required, that can be provided from another GFCI device upstream. Also, having a GFCI device past a light switch is a bad idea, so if this is sourced from a wall switch, no GFCI here. That's a hardware limitation, not Code.
You also can't put a GFCI where shorter members of your family need a ladder to reset it. How will they see to work?
Also practical, you don't want to put any GFCI device where it'l be exposed to rain, spray or condensate, so an upstream/more indoor location is more appropriate. You wouldn't charge your iPhone out here!
The next question to ponder is whether you'll be touching or contacting this light. Bathroom lights are generally grounded and garage or hi-bay lighting is usually unreachable. If frequent contact or no ground, there’s a practical argument to be made toward GFCI protection, just because it "takes off the table" any risk of electrocution.
Now ceiling receptacles are perfectly normal for lighting. Wiring luminaires via cord and plug connection is perfectly allowed due to NEC 400.6.
The code section that deals with GFCI protection for residential dwellings that would apply to you is 2018 NEC 210.8(A)(11) which specifically is for indoor damp or wet locations.
The extra level of protection a GFCI offers is worth it as long as you don't create other problems in the process. For example, the GFCI should be accessible without too much effort. For this reason I'd recommend installing a GFCI switch in place of the regular switch. If no switch, then GFCI breaker or accessible outlet upstream.
The code also requires AFCI protection for all habitable rooms so you might as well install a DFCI ( GFCI + AFCI) breaker and be done with it.
Anytime there is a higher chance of rain, water splashing, possibly condensation you should always install a GFCI; Ground Fault Circuit Interrupter...they should always be in a bathroom or a kitchen. Not a place to save money that is for sure! These things should be installed by a licensed and bonded electrician to protect your home owner's insurance!