Well, there is so much that's different between Mark King and Larry Graham/Louis Johnson - in terms of the musical role they play in their bands - that it's difficult to rate them comparatively. All three are thumb guys, but that's where the similarities end.
IMO, Mark King is not exactly a bass player. His job in Level 42 is to sing, and as a soloist playing the bass. Most of his bass lines are ornamental, percussive, and as such (as you say) his bass EQ is done in such a way that it does not serve the normal purpose of a bass, at least not well. In most cases, the "bass" role in Level 42 music is served by synth lines, and Mark doubles those synth lines sometimes. Perhaps a comparison with Stanley Clarke or someone who records and tours with a second bass player playing underneath them would be more apt.
All that said, what I have liked most about Mark King is that there is really nobody out there like him in the sense that there's nobody contributing to their music on the bass in the same way he does and has done.