The Valk Helm (or Forsaken King's Helm, as it is also called in the server,) only renders you immune to freeze if you're wearing the whole set, with the exception of the Shield. (Helm, Armor, Cloak, Boots)
A Clown has ways around most cards/equipment that renders a user immune to freeze (except for one item in which myself thinks is too advantageous) by stripping (High DEX) or breaking (Tarot, or armor-break cards/equips,) but that is easily countered by FCP.
In 1v1 situations I find it more "useless" but during WoE, there's a lot of players who are not as ready as you think they are, therefore no FCP, or worse, wearing equipment without freeze immunity.
Overall, Frost Joker is still one of their useful, conditional skills. I think it's alright.