Using generic book covers offered by KDP labels your book as amateur and unprofessional. If your intent is only to sell it to friends and relatives that’s fine, but if you want to compete in a commercial market, it’s not going to fly. A generic or poorly designed cover sends the message you don’t care enough about your work as an author to package it correctly. Furthermore, the cover is a potential reader’s first introduction to your story. If it doesn’t catch their eye, it’s doubtful they’ll have any interest in reading it.

I’ve read some books that were horrible in that they desperately needed editing, yet had luscious covers, showing the opposite can also be true. If you want to be viewed as a professional, the quality of both should be top-notch. There’s a lot of competition out there and you don’t want to give people an excuse to pass you by.