KISS is an acronym for “Keep it simple stupid” (notice no comma).
Stupid does not refer to the user. To quote Wikipedia:
The principle is best exemplified by the story of Johnson handing a team of design engineers a handful of tools, with the challenge that the jet aircraft they were designing must be repairable by
an average mechanic in the field under combat conditions with
only these tools.
Hence, the “stupid” refers to the relationship between the way
things break and the sophistication available to repair them.