Your experience shows the exact opposite of mine regarding the code quality. Since we have adopted an agile methodology, I and all the team can attest that the code quality improved a lot. Code reviews and team ownership helped us build a common knowledge on the code and best practices, and everyone is conscious that quality is important. And everyone participates to enforce it.

Now, what I can see about your experience is that your problem is real, but maybe you did not identified the correct cause.

If your team correctly implements the agile principles, you will be able to discuss this point in a retrospective (or something like that) and find a solution together to tackle this issue, and don’t repeat the same error twice.

If you do not have such process, don’t blame agility because you’re not doing it.