That's true. Expensive cards are definitely usable, especially if you have cost reducers in the deck. Heck, there's a Cthulhu Event that lets you put Cthulhu into play at no cost if you sacrifice 3 Cultists. I've seen the big green guy drop on turn 2 using that trick.
In fact, I was talking with some guys a while back who said that an Ancient Ones deck with LOTS of cost 5-6 characters was pretty dominant in their area.
So while there is some "conventional wisdom" about cost curves and such, it's a lot like the Pirate's Code. More guidelines than actual rules.
And as for the Mutant Spawn… I don't think he counts as a monster yet, at least not until he learns to walk. I'll take your advice though and cut down on the Conspiracies :)