Camera quality is the same within the same app on either.

Pixel Camera can be used on GrapheneOS with full features and photo/video quality if you want it.

GrapheneOS Camera has support for HDR+ on Pixels for regular photos and has Night mode too. It has EIS and HDRnet for video recording. It has a single exposure slider rather than their dual exposure sliders. It uses each of the cameras via zoom level / light level in the same way. More advanced features and configuration are being added to it over time.

Pixel Camera has more features and the HDR+ it uses is more aggressive which makes the photos look higher contrast than a more natural look.

GrapheneOS' own camera app doesn't have Google's propriety processing, so it's not as good as stock, but with Pixel devices, you can install the original Google Camera/Pixel Camera app and get the original camera quality.

GrapheneOS Camera does have hardware accelerated HDR+ and Night mode along with HDRnet and EIS for videos on Pixels.

Photos in Pixel Camera look different because the HDR+ it uses by default is more aggressive resulting in higher contrast but a less natural look. Both are using HDR+ with hardware acceleration though. Videos are more similar and Night mode photos likely are too.

But I think Pixel Camera does some kind of sharpening post-processing, because I get blurry images sometimes with GrapheneOS Camera while almost never with Pixel Camera.