When my kids were in diapers and it was really hot out yes I would take them out in public wearing just a diaper and their shirt.
They would usually take off their pants or shorts anyway when it was really hot so I figured I would just let them be comfortable.
My kids went out into public at parks in the grocery store, Walmart etc riding in a shopping cart or walking alongside me wearing nothing but their diaper, shirt and shoes.
It was so cute and I wish they were still young enough to be in diapers like that again.
I see many other kids out in public in just diapers on hot days and It's perfectly fine because the diaper itself is probably making them hot enough as it is.
Some people are disgusted by a toddler being in just a diaper in public but I'm not and I find it cute.