Well, in the book, Hazel Grace describes her features as thus:
“I was wearing old jeans, which had once been tight but now sagged in weird places, and a yellow T-shirt advertising a band I didn’t even like anymore. Also my hair: I had this pageboy haircut, and I hadn’t even bothered to, like, brush it. Furthermore, I had ridiculously fat chipmunked cheeks, a side effect of treatment. I looked like a normally proportioned person with a balloon for a head. This was not even to mention the cankle situation” (Chapter 1).
Is this really what Hazel Grace looked like? Probably not. Hazel Grace is what is called an unreliable narrator. Since she has suffered from cancer for years, she is very self-deprecating.
While Hazel makes sure the reader knows about her "pageboy haircut" and "chipmunked cheeks" and "balloon for a head" with "cankles," she also lets the reader know she has tubes that run into her nose and rolls around an oxygen tank. In addition, we have a clue of what Hazel really looks like from how Augustus responds to her. He calls her "beautiful" and says he enjoys "looking at beautiful people."
Side note: Hazel Grace is based off of a real girl, Esther Earl, who became good friends with John Green before she died at 16. (Just search YouTube for "Esther Grace Earl.)