I would say yes as well. It just talks about "figures within 3 spaces," so monsters count as figures.
Thematically, it is a cloud of mist, enshrouding everything inside of it. There's no reason it would make Heroes more difficult to hit but wouldn't do the same for Monsters.
Of course, the solution for the Heroes is to have your Spiritspeaker go last, so it's irrelevant.
It is relevant for two reasons.
1. The card will not refresh (so the effect will still be in play) until the Spiritspeaker is activated on the next turn.(Yes, you could activate the Spiritspeaker first the on the following turn to remove the effect but if you to used it again this turn it would definitely affect the heroes)
2. It might be tactically disadvantageous to move the Spiritspeaker last.
So, yeah you can sorta-kinda get round it but it is far from irrelevant.
The best way to get round it is probably to keep just inside the mist and attack out.