Athens tour

In Athens, hop-on hop-off buses can't access major sites, these sites are mostly in pedestrian areas. But you can get off the bus at designated stops and walk the remaining distance.

Taking a hop-on hop-off bus that travels along the main avenues in the middle of traffic and pollution with the goal of staying in it isn't really worth it. You might as well take a public bus from which you'll have a better view for €1.20.

No, a Hop on Hop off bus is not a good idea.
Athens is very walkable but the heat may be a problem. Walk as much as possible and use taxis to go from place to place farther apart. .

Yup
Saw what looked like hot and unhappy riders on those buses
Athens is walkable as was noted
But get out early, wear a hat, and bring plenty of water
It was hot

We visited in September. Hot but not miserable.
My advice is to use Rick Steves’ Audio Europe app and download the Athens Walk to do your own self-paced tour. It was a great way to spend a day seeing the highlights and stopping for a beer or lunch at a shady taverna or spending extra time at any site we chose. Honestly, I would spend some time at the Acropolis Museum and consider skipping going up to the Acropolis altogether (sacrilegious, I know!) Honestly, it was my least favorite part about Athens… it’s beautiful from afar and lit up at night, but when I got up there, I didn’t find it as fascinating.