It's not great poetry, but it's fine.
A "simp" may be a person, but it is also a thing.
Think of plumbers. They're people but they're also a profession (at least in abbreviation).
"This uniform is usually worn by plumbers, [one of ]which I am."
"Who" wouldn't work either here or in your example, as "who" would signal a particular flesh-and-blood person rather than category of person (whether it be a profession, an ugly intellectually category or something else).
Neither would "that", because it's not in a defining clause (one which identifies the particular person or thing you're talking about) but rather a non-defining clause (one which provides a new piece of information on an already identified person - in my case, me ... and in yours, 'he').
There's a joke about a double question that's reputed to have been asked far too often in the bad old days in Belfast: "who are ya, what are ya?", which is an inquiry about your name and background (including the neighbourhood and family you grew up in, where you went to school and what you do with your day).