Even online, it's possible to punish a blocked superman...but if the lag is exceptionally heavy, I can't blame you for having difficulty.
There are two ways you can punish a blocked superman. The first is attacking before Raiden lands the ground. Sure you will be inflicting less damage as per the mechanics of an anti-air combo, but if you'd rather not risk them landing on the ground and being able to block in time, that might be the better option. Alternatively, if you feel confident enough, attack Raiden after he lands from his blocked superman.
With Sub-Zero, I'd go 2 2 into Freeze Ball and then end with b1 2 1, or there's 2 1 2 Freeze Ball, b1 2 (whiff b1), 2 2 Cold Slide.