Is it okay to dress up as Hazel Grace Lancaster for a Halloween party?

No, it IS offensive. First, if using a cannula is the only "recognizable" part of a costume, you're really just treating someone's illness as a costume. Illness isn't cute or fun, I know this from my own personal experiences. If it's the only recognizable aspect, then maybe you shouldn't dress up as them. An illness is not a costume, and you would be treating Hazel's and anyone else who has had to wear or presently wears a cannula as if that is their only defining characteristic. It's just really offensive, and Halloween is no excuse at for this kind of behavior. I don't care if you think you aren't mocking her, you kind of are. You're appropriating illness and disability, and that is mocking. Here's a tip - if you have to ask if something is offensive, then it probably is.

For me, I don't think that's offensive, I mean one point of Halloween is to dress up as a character or initially a scary one but that's alright. Also, think about it, you're NOT intentionally mocking her, you just want to dress up as the character.