That depends entirely upon the type of camping you are doing, and where you are camping.
If you are camping in the Northern States, you can probably get away without an air conditioner in a pop-up. If you are planning on visiting the Southern State, you'll wish you had one if you don't.
We had a pop-up for 6 years and it did not have an air-conditioner. We came from tent camping, so didn't think too much about not having air. Coming from Indiana and doing mostly Indiana camping, and doing early Spring and late Fall camping, we found having a furnace was more important.
If you are a tent camper, you will probably never miss having an air conditioner in a pop-up. But once you have one, you'll be spoiled and won't be able to live without it.
If you camp under trees, in shaded areas, Northern States, Wisconsin, Michigan, Indiana, Ohio, Pennsylvania, and New England, across to Iowa, etc, you can probably get by without one. Of course, like in a tent, it can get stuffy if there is no air moving. If you camp in direct sunlight, with no shade, even in the Northern States, it can get pretty hot inside a pop-up.
Yes, you can open the flaps and set up fans, but then ... if you can see out, everyone else can see IN also! It's not very private. To get the privacy, you have to close up the windows, so then the heat builds. You can open the windows part way, but may not get a breeze on you.
Really, is just a matter how acclimated you become to the outside temperatures.
If I were to do it over again, I'm pretty sure I'd want my pop-up with an air conditioner (second time around). But, absolutely no foreseeable plans anywhere in my future to ever going back to a pop-up.
Definitely, if you go to Gulf Shores Alabama in early August ... YOU WANT AIR! Don't ask how I know!