Admittedly, I mostly know Sinister from the Claremont run (where even with how he portrayed him, he and Simonson laid it on thick that the bully from Scott's childhood was Sinister's true form) and the animated series. The former became more interesting and creepier with the knowledge of the behind the scenes planned origin in mind. And I believe Nathan, even stuck as a physical 12 year old, was intended to be old, so his Victorian background could still be in place. From the animated series, I thought he kind of sucked even with his origin episode. "Mr. Sinister" is an inherently silly name with an OTT Goth Colossus looking design, it's hard to take a character like that completely seriously if he's intended to be Machiavellian. It actually makes sense coming from the perspective of an adult mind still trapped in the body of a child that isn't keeping up with the rest of him that is putting on airs.
But my X-Men collection tends to stop right when Claremont leaves in 1991 and picks up again with Morrison. The 90s are a decade I don't want to touch with a 10 foot pole (I'm sure there
s good stuff in here, but too much crap surrounding it), so I have no attachment to the origin Sinister ended up getting. I don't really have a horse in this race if Hickman is going to be messing with it or pulling inspiration from Claremont's original notes.