I'd say that this recommendation is a sign of an SEO company looking for reasons to show you why you need them to improve your site, and is not an issue for your website's performance or rankings.
First of all, the report is obviously generated by software, and not a human evaluator. It failed to notice that the third stylesheet is for "print" only, which means it would have no effect on speed for the user or search engines.
Second, the number of stylesheets used is only a significant speed issue in terms of the overall number HTTP requests required to render the page. It's a gross exaggeration to say that extra stylesheets or "use of CSS code within the body tag" have a significant effect on page loading except in extreme cases.