How the interior of your book looks is as important as its cover. Some readers will put aside a book that has poor formatting because they either find it too difficult to read or it’s hard on the eyes.
The conversion process used by KDP might make changes to your ebook’s format. Always preview your ebooks once they are uploaded. The online preview took is useful but it might not catch everything. If you can, download a copy of your ebook and look at it on your Kindle or whatever ereader you use. In that way, you can make sure things like your table of contents and any links inside of your ebook work.