why's the army of olympus all undead and monster?

 maybe i'm just not acknowledged on Greek myth but I was expecting to fight with somewhat holy/angelic theme enemies.
just kind of weird me out seeing all the cyclops, siren and other monster from the myth fighting for Zeus.
   the only thing that seem fitting were the statues.

Nope.

how much greek mythology do you actually know about?

There's not really anything holy or angelic about the Greek gods.