I am with LFN that Curses are already pretty bad but let's follow through with your idea of making them worse.
I think that something like Hex has an impact whenever the game is junking-intense, probably due to Looters and might end with a 3 pile ending and a low number of VPs. The other instance a "make Curses worse" card is gonna achieve something is when the Curses are split and (nearly) out in the endgame, you remember the count and, when it is in your favour, decide to hurt the other player. In a 2P game that is either gonna be a VP differential of 2 in the case of a 4-6 split or 8 in the case of a 3-7 split (ignoring trashing for the sake of simplicity).
In the first instance Hex is like Island that doesn't set aside anything but itself and in the second case Hex is like Distand Lands.
So I think that one issue of such a card is that it is only about VPs and not about junking. That's not per se bad but it is only about VPs in a limited set of Kingdoms., quite often something like Hex might not be bought.
Perhaps you might wanna try something like handing out two Curses for additioanl junking or implementing it as an Event for immediacy.