Furniture up against a plug is of greater concern than a curtain obstructing the receptacle.
Think about this. We install receptacles in master bedrooms. There is a night stand each side of the bed. The receptacle is often in the middle of the wall behind the headboard. A young couple who is active at bedtime is slamming that headboard into the plugs for hours on end.
In your case there is less airflow and maybe accumulation of dust. While that's a concern, we can only go with the codes (at a minimum) and the protections the codes give us. If you're that concerned, and it's not on AFCI, make it AFCI so the dust and curtains don't catch fire.
I actually think you're safer with the curtains there because there is less chance of the occupant pushing furniture up against it so hard that it damages a plug.
