Goalkeeper Ashdown allowed Lee Frecklington's shot to squeeze through his grasp for the game's opener and Norwegian striker Huseklepp, who created both of Pompey's goals, turned the ball into his own net right at the death to hand Posh the win.
Despite Huseklepp's error costing his side a point, Mullins said: "Erik's got the pace of the game and he's feeling alright, then that goes and happens to him.
"He just needs to keep creating goals for us and the fans helped him by singing his name at the end. That was a nice touch.
On Ashdown, Mullins added: "He held his hands up and he apologised. It's hard for him but all the players are behind him and he knows that."