First something that is probably not it. Is she broody? When a broody is out for her daily constitutional, she usually walks around puffed up. She is usually fairly active though, eating drinking, pooping, and just in a hurry to finish her business so she can get back to her eggs. I have had some broodies when the weather is really warm stay off the nest for a long time and socialize with the other chickens but they are usually still fluffed up and making a pocking sound. And I have had some that walked around like that but were not really kicked over to full broody mode.
Sometimes when that are dust bathing they’ll lay around fluffed up. They are not necessarily active then either.
If she is lethargic over a time period, it’s probably not a good sign. Chickens fluff up and stand around when they don’t feel good. If it only lasts a few minutes I would not worry about it, but if it goes on for an hour or more, I’d be concerned. It could mean mites or lice, maybe worms. They act that way if they have coccidiosis or other diseases or if they are injured.
I really don’t know if it is a problem or not, but it is certainly something worth watching.