While they might look the same at first glance, there is a big difference between a “derped” fluffy and a “pseudo-derped” fluffy.
A “pseudo-derped” fluffy is simply a fluffy with Strabismus, or misaligned eyes. Just like in humans and other animals, Strabismus in fluffies has several different types, and can be caused in several different ways.
But while a fluffy with Strabismus will have problems with its vision and may look “derped”, it will still retain the mental capacities of a normal fluffy.
Meanwhile, a “derped” fluffy is a blanket term for any case of a mentally deficient fluffy which also has Strabismus.
Due to not actually being a single condition, a derped fluffy could be one that developed both Strabismus and severe brain damage from head trauma, or one with both Strabismus and diminished mental capacity from birth.
Overall, the easiest way to tell the difference between a true “derped” fluffy and a normal fluffy with Strabismus is to wait for it to open its mouth.