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Aside the fact pylons mounted on top of wings look bizarre, is there a reason not to? Top-mount would seem to reduce FOD, & shorten gear legs (& so weight).
I understand keeping the engine away from wing turbulence is good, so a strict over-wing might not be ideal. What about a forward-swept? Or back-swept? That is, more/less a standard Me-262 or 707 pylon, mounted on top, instead.
I also realize keeping jet efflux away from the horizontal stabilizer is desired; T-tail might be needed.
So, beyond esthetics, why not overwing instead of underwing?