Should I get an outdoor extension cord instead of using an indoor one outside?

Yes. Indoor extension cords aren’t rated for UV, moisture, or temperature swings, and “dry weather” doesn’t guarantee dew or condensation won’t sneak in. LED lights draw almost nothing, so amperage isn’t the problem — it’s the insulation degrading or moisture causing a short. The price difference is about $3, so it’s not worth the risk. You’ll sleep better knowing the cord is actually built for the conditions.