How come Norman Osborn became so popular since he was prooved to be the Green Goblin in the first issues of The Pulse and MKSM, and he killed a woman journalist ?
Or did I miss something with Norman cleaning his reputation ?
There's nothing in particular that's happened in Thunderbolts that has made Norman more popular (in fact, if the Marvel universe public knew what happened in there he'd become infinitely un-popular).
But it appears they're using the events spinning from the climactic Showdown with the Skrulls where it looked like Tony Stark was "running away" while Norman was "heroically" continuing to fight the good fight, to have Norman become an American hero and move his crazy Goblin butt up the government power ladder.
As for why he seemed such a lovable guy in front of the New York crowds in ASM's "New Ways to Die", I can only theorize that he used his many government and business connections to clean up his image while he was running the Thunderbolts (like how Dexter says the Daily Bugle "said [Norman] was the Green Goblin" and also issuing a full retraction and apology on behalf of the new DB).