We're a design company.
I wonder if adding a list of client websites we have completed as a section after our site map webpage (rather than XML file) will hurt our SEO? (or even help our SEO?)
They would be non-follow links.
There is no reason to include said section in your sitemap. The purpose of a sitemap is to help crawlers in crawling your site, especially if your site updates its content regularly or if it has archived content that is isolated or hard to reach.
The urls are of different websites so there is no sense in including them in your site map.
You can put the external links in your internal portfolio page. No follow links should not hurt your website. XML sitemap is dedicated for internal links.