What does i'd like to have been an eardropper on the wall mean?

Nothing. I suspect you're confusing several phrases.

Eavesdropper - the eaves are the top parts of a house, where the gutters are - if you're an "eavesdropper" it implies you're hanging from the eaves listening in at the Windows - in other Words, you're listening to conversations you shouldn't be listening to

"A fly on the wall" - if you'd "like to be a fly on the wall" you're saying you'd like to be somewhere hidden where you can see and hear what's going on in a private conversation, as if you were a little bitty fly hiding somewhere in the room

Eardropper - this is a medicinal device used to instill drops into your ears when you have an earache or wax buildup