While there is already a Code Review site, there aren't many questions there about shader code, and I feel that those kinds of questions would attract more attention here.
Should reviewing shader code be on-topic?
I'm tempted to say yes, due to the fact that there's only about 5 such questions on Code Review. But on the other hand, that site is the correct place to ask questions on how to improve working code. As an active user of CR, I'd also like to see more questions relating to shaders over there, so I'm a bit biased.
I think if the code is working and the user is looking to make it cleaner / better / more concise, It belongs to Code Review. Questions asking for suggestions on how to improve efficiency or performance of shaders would also be a fit for CR, but I doubt these questions would find a lot of help on that regard over there at the current stage. The site just doesn't have enough reviewers that are comfortable with shader optimizations.
So disagreeing with @robobenklein's comment, reviews looking to improve performance are a better fit in here, optimizing shaders is an expert area. Making code more readable / maintainable / scalable is definitely a Code Review topic for any language.