Mysterio has this power

MIND-NUMBING GAS: Opposing characters that occupy a square with a hindering terrain marker placed by Mysterio at the beginning or end of an action can‘t use Indomitable, Super Senses, or Willpower until the end of that turn

Whose action? Is it anybody's?

If it is the character in the cloud's, then not being able to use Super Senses has no value at all.

If it is Mysterio's, the only effect is to nerf Super Senses or make some characters more vulnerable to Incap

If it is anybody's, then the only way to prevent its effect is to pass your entire turn for your whole team

Sounds like any action, anybody's turn.

And it would effect other decisions as well... If you no longer have willpower, or indomitable then it will make you think twice before pushing. Not to mention if you cant use those powers then it could render those figures unable to use other powers (Feats come to mind).

Correct.

MNG has an effect during Mysterio's turn: target cannot use Super Senses. (take THAT, Spider-Man!)

During your opponent's turn, if a character affected by MNG wants to Push, he will take Pushing damage unless a game-effect other than or Willpower is in play.

Can't wait to dig mine out and play him with an Utility Belt filled with Gas Pellets!

He can't use the gas pellets, because he already has access to smoke cloud.

What he said.