I think it's a fair assumption.

Vs a fish though, even if he has a weakish range I think we can still get his stack (or a sizable portion of that) if we knew he overplays hands. An example I can think of is if villain min-3 bets any Ax hands and the flop contains an A and he overplays TPNK then we can probably get all the money in. In a way, there's more opportunity for us for our set-mining to succeed.

This could all be wrong of course, just something I'm thinking about.

I do get your point about UTG though, so this is probably a wrong example of when to set-mine given less than 20:1 odds.