What are those mutators in EG2? Something like achievements?

Although they are unlocked in the classic achievement sense, they are more than that, namely do they have an effect upon the gameplay if you activate them. I fired my copy up to recheck what they do, here is the list (I have already unlocked all of them so you might not see the names, they are given in the order they appear in the menu):
· Speedrunner overlay - gives you a speedrunner overlay.
· Super meat goat - replays all of your actions after you complete a level (slightly sped up, though).
· Grim darkness - reduces the amount of light in levels, making everything not directly in contact with the goat or another light source completely black.
· Unseen doom - hides reapers visually. This leads to fireballs out of seemingly nothing.
· Spectral goat - makes you invisible when you move (this might be purely visual - I think reapers will still try to shoot you).
· Infinite jumps* - gives you infinite jumps.
· Explosive mouse* - the mouse returns in an explosion every time you recall it.
(Entries marked with an asterisk is considered cheats and marks you save game).

Cool! Thanks.