It depends on where you live. If you live in a humid climate, leaving your clothes overnight increases the chances of the clothes remaining damp. If they stay damp, then they can host mold and mildew, which can make them smelly (source).
It also might rain overnight, which would bring your dry clothes back to square one.
You might also have issues with birds and insects which live outside. They can dirty up or ruin your clothing.
In general, we recommend taking your clothes back inside as soon as they are dry and not leaving them out overnight.