Italic is an adjective, emphasizing its current form, while italicized is a past participle, emphasizing what was done to it, as user3169's comment suggests. Both are reasonably correct, but it's fairly clear from that 2-gram that the former is more common; we usually care more about the fact of its appearance than the implication that someone went and made it that way.
An example in which you might use the latter is if someone has taken the time to italicize certain words in an existing document to pick them out. You might say, "Pay careful attention to the italicized words, which signify …" or perhaps "The italicized words in your papers are examples of purple prose I've found in your writing; you should be careful to limit your use of those accordingly." In neither case is this essential (they certainly can be said to look italic if they have been italicized), but it might be preferable.