In my opinion, Julia can be called apolitical because her hatred for the Party is not based on any ideology. Instead, she only hates the Party because of the impact that it has on her personal ability to go and do what she wants.
When Julia is talking about her attitude towards the Party, she does not think about it in philosophical terms the way that Winston does. Instead, she hates the Party because she wants to go out and do things (like having sexual affairs) that she thinks are fun.
I think that you can see this in the following quote from Part 2, Chapter 3:
She hated the Party, and said so in the crudest words, but she made no general criticism of it. Except where it touched upon her own life she had no interest in Party doctrine.
Julia hates the Party because she sees through it to what a corrupt fraud it is, not because of any adherence to a particular politics. She's a materialist, interested in her own physical and psychological well-being, and she understands that the Party blocks people from achieving either one. She understands that the Party wants to use her energy for its own ends, and she resists that. She, rather than Winston, perceives that the Party wants to prevent sex not only to keep people miserable but to channel their energy into achieving Party goals. She wants her energy for herself. She also doesn't care whether the Party changes history, and she works from the a priori or beginning premise that the Party lies.
Orwell leaves it an open question as to whether Julia's pragmatic self-interest or lack of any illusions about politics is better or worse than Winston's desire to believe in a dream of a better and more truthful world. It's also an open question as to whether Julia is truly "apolitical": can anyone so cynical about the state not be seen as having a political position? Certainly Big Brother sees her as a political subversive, even if she has no coherent politics.