As others have suggested, it only takes a 10 minutes to plug in a hair dryer or space heater to the suspect outlet and see if it holds. If the GFCI is bad, it will likely trip pretty quickly. If it runs a hair dryer for 10 minutes, the issue is in your lighting and the GFCI is just doing its job. GFCI outlets work by detecting leakage current to ground. It doesn't take much current leaking to trip one (4-6mA). They can trip with even tiny loads attached to them if that load has some leakage current to ground. The other good suggestion was to run an extension cord from another GFCI to your christmas lights and see if it trips too. Based on what happens, you can swap the outlet or not.

If it turns out your outlet is good, then look for any place your lights might be grounding out. Most christmas lights don't have a ground wire so they would need to ground to something in order to trip your GFCI. If it was a bare wire short, your light string would burn up like a fuse so it's likely a crimped wire with the insulation being crushed by something metal or something wet. A high-resistance short to ground will trip a GFCI but not trip the breaker behind it. The comments about moisture are right on as well since water can break down insulation and allow current to pass through it.

I've got outdoor GFCI outlets that are 20 years old and still function perfectly. One of them saved my life about 6 years ago when I stepped in a puddle that had a cord in it just as it tripped out. Assuming the GFCI is the problem ignores the fact it could be doing exactly what it was designed to do. Stay safe.