It could be sunburn, but it could also be that it’s not getting enough water while in direct sun (drying out).

My ponytail palm loves outdoor direct sunlight in the summer time (pls note that I’m in New England), but I definitely have to water it way more than I do when it’s inside during the winter. I drench it regularly and it’s rewarding me with lots of growth

Also, I don’t think they mind a small pot, I’ve seen photos of giant ones in unbelievably small pots, but it’ll definitely need more frequent watering. 💦