Yes it would be odd/bad form to ask an interviewer to sign an NDA. Sort of a segue here, but I’ve been an interviewee brought in to discuss a design job where the employer could not share any of their work with me or tell me who any of their clients were. I knew what type of design work that they did, but in order to determine if I would be qualified and/or enjoy the work I wanted to see some examples of their projects to get a better feel, if for nothing else to engage in some interview dialogue. They refused, I offered to sign an NDA, they even had to refuse that due to stipulations in their client contracts. I declined their offer.
The point is if you have time gaps in your portfolio due to things you cannot share, I’d recommend being well prepared to point that out, but also be able to discuss your role and thought processes for those projects without divulging any confidential details, if you can relate those experiences to the projects that you can show then even better, it will show how you can apply that experience to your new employer’s projects as well. The company that I interviewed with didn’t do such a great job in articulating at least what it was like to work on their projects or speak to process and it made me uneasy about the job, the same thing can make a hiring manager uneasy about a candidate.