[Userscript] Community Mnemonics 2 (WKCM2)

Just to clarify, do you want to download all Mnemonics from WaniKani, or just the ones added by this Userscript?

If it is just the latter, just go to the Google Sheet, where all Mnemonics are stored: WKCM2DB - Google Sheets and go to File → Download → .csv / .tsv. Since it stores JSON data, which contains commas, you might want to choose .tsv

If you want the ones from WaniKani, I am not aware of a program that does exactly that. However, the API allows you to access the Mnemonics: WaniKani API Reference

Edit: One quick Google search later, I found this script, which should do what you want: [Userscript] Wanikani Item Inspector