If you’re playing lever harp, a carpet may be fine, but avoid any busy patterns because they’ll drive you crazy when looking at the strings. If you’re playing pedal harp, you need something more rigid because carpet over grass is lumpy and your heels will get caught. You need a reliable surface for pedaling.
I keep a piece of 3/4 inch plywood covered with green astroturf in the back of my van. It’s just long enough for the harp and bench and just wide enough to accommodate the music stand, too. It blends in with the lawn and can be brushed off or even hosed off afterward if needed. It really saved me last year when I had an outdoor wedding where the thick, new sod was soaking, squishy wet after heavy rains. A carpet would have absorbed the water and become as wet as the turf was.
Another good option for pedal harp (but only if it’s a country club or some such place) is to just ask them to put out a 72″ round banquet table with the legs folded up. Makes a great stable platform and plenty of room for bench and stand, too. Obviously this isn’t an option at a private home or a small wedding in the park, but it works very well when you can do it.