A young baby or even a toddler that's still wearing diapers full time and not potty trained it's okay to let the baby or toddler wear just a diaper when around the house or in or around the yard.
But when the baby or toddler goes out into public spaces the baby or toddler should have clothes on or at least a onesie over the diaper because even in the summertime the baby could get cold when going into air conditioned places.
I'm not offended either way though if a baby is in public wearing just a diaper so do whatever you and your baby feels comfortable doing.
If your baby is more comfortable during the summer months wearing just a diaper then it's okay.
During the fall months or winter the baby should always be clothed to keep warm but in the house it's okay for the baby to go around the warm house wearing just a diaper.