Most everyday clothes are just fine for everyday riding.
When you're doing specialized riding (e.g., training, racing, for sport), clothes designed to keep one comfortable and minimally chafed are advisable too.
Now if someone suggests that one absolutely needs cycling shorts and a team jersey to ride an IGH city bike to the store and back...well, I can tell you the Freddiot
isn't the person who is content using everyday clothes for this task.
-Kurt
P.S.: The safety nannies have yet to come up with any scientific evidence that sandals are a problem while riding. Wear them with pride, knowing that you're pissing off someone in their SUV who thinks that your sandals - not their two-ton battering ram - is somehow the biggest threat to human life in the vicinity. Irony can be sweet and on your feet.