While I get why asking to view a codebase could come across as sounding like someone only wants to work on neat and tidy solutions, I don't hear it this way.
It's very normal and human to have preferences. It's OK for a prospective employee to ask questions about the working environment. There are a number of valid reasons to try and gain more in-depth signal on technical aspects of the company that don't mean someone is a snowflake.
There are a lot of ways to gauge this kind of signal without viewing a proprietary codebase, as others have mentioned in this thread. The questions posed in the comment you replied to are some great examples of how to do this - I think these get to the core of the matter more effectively than looking at code.