As someone who freelances, this is just the game. She agreed to the pay and did it. And now that the film has done well, I'm not sure what she expects. Retro active pay beyond what she agreed to? She has her name attached to a highly successful film. She could market herself for future projects where she could make more.
Instead, I think with this attitude, she's going to make future producers think twice before they hire her despite her work on this successful film. I know I would think twice if her resume and this social media post was in front of me. If there was a candidate of equal or even slightly less quality than her, I'm still going to consider them in relation to her in hiring.
The trick to success is to always be looking ahead. How can what you have done be a stepping stone to what you want to do. She could have reframed this post even. "I was art director on this highly successful indie film but I also wore a lot of other hats. I'm proud of the work I've done. This is the nature of indies." Instead of sounding like a complainer, she could have sounded highly dedicated. That gets you more work. That gets you on bigger projects. That gets you more pay.
Honestly, this attitude comes off as failing in personal character to me.