I see kids like that a lot at parks and events as well and even around my town.
As long as the weather is warm or hot and the child is comfortable wearing just their diaper, shirt and shoes or just a diaper in public, I see nothing wrong with it.
Even my kids would go to the park sometimes in just their diaper, shirt and shoes even when they were 3 years old.
I have a 3 year old boy that is still in diapers and he goes to the park and plays outside like that in our yard as well and even goes into the store wearing just his diaper, shirt and shoes when it's hot out.
If somebody says something to me, It doesn't bother me.
I just prefer that my kids and other kids are comfortable and if they're comfortable in just their diaper, shirt and shoes then that is how they should dress as long as the weather is right for it.