For a universal, objective idea of simplicity refer to Technical Simplicity.

However, when we use the word simple we often mean in terms of how the given thing appears or how humans interface with it. This is what I call Interfacial Simplicity.

The two ideas are not mutally exclusive, but it is definitely possible for a something to have only one of the two properties.

Perhaps different words could be used to describe the different ideas. One idea:

This semantic metaphor implies another possible distinction