How do I escalate a Mail UI/usability bug to Apple without having to include sysdiagnose data?
Assuming you're referring to the Mail app on macOS, you can submit a bug report via the Mail feedback page.

In the Feedback Type drop-down list you'll have the option of selecting Bug Report as your feedback type. There's also a Design/Ease of Use option that may be relevant in your case.

You can also access Apple's main Product Feedback page if your feedback is about something else.