A cookie cutter is literally a metal 'template' to cut cookies out of dough.
In the case of a gaming-build, it's a standard build, according to a well-accepted template.
Compare for instance with a Diablo II 'Hammerdin' or an 'Lightning Sorceress' or a 'Summoning Necromancer'; these are also cookie cutter builds.
If the other Mythos-related terms are eluding you too, I would advise you to check the Mythos forums, to find out what a Pyromancer is, or Meteor, or Piercing Barrage, etc, etc.
Regards,
Patrick.