Quote Originally Posted by Curmudgeon View Post
Without divination magic or some other way of determining whether an apparent creature is actually a figment, the description of Mirror Image requires that Cleave functions identically, to your sight and hearing, with those figments. If you had Cleave and your weapon behaved differently when destroying an apparent creature vs. destroying an actual creature, you would be violating the above requirement of Mirror Image. Only a detection means other than vision or hearing, such as divination magic, would allow a difference. Lacking some such other sense, the observing warrior with Cleave must see and hear the same effect on their weapon after dropping a figment and after dropping the spellcaster; that is, the weapon will Cleave through to attack another target in reach.
How it is perceived is irrelevant for the rules. Just like for example bane(undead) weapons still function even if you don't know the target is actually undead.

It is literally stated that it requires the target to be a creature. You seem to confuse the rules with how you'd like them to be.