I must say Persians without beard were unheard of at the time. And Xerxes, who was a great emperor, was not like that.
One thing to note, is that the Persian empire which lasted 100s of years until Alexander was one which was considerred beneficial by most as regions were empowered and much of the power was decentralised.
The Greeks were a bunch of loosely connected city states.
The battle of Thermopalie (I know the spelling is wrong) was made possible due to the geographic loaction which did not allow the 300 soldiers to be encircled. The valor of the Spartans is not under question and they had the best hoplites of their time.
Anyways, the graphic novel is pure fiction and should be taken as such. However knowing the general movie going audience, you just know some will take it as history rather than fiction very loosely based on an historical event.