Setting is undocumented. Judging by the source code, it's only purpose seems to be controlling from what hook MediaWiki:Mobile.css is loaded. Please justify why you want to set this to true on your wiki or this will be declined.

The reason for setting $wgMFSiteStylesRenderBlocking to true:
Make MediaWiki:Mobile.css effective in mobile theme to customize the CSS for mobile theme.

The MobileFrontend page at mediawiki.org recommends adding the following to your MediaWiki:Mobile.js if it doesn't load:

mw.loader.using('mobile.site.styles')

Actually, setting `wgMFSiteStylesRenderBlocking` to true would make styles inside MediaWiki:Mobile.css become render-blocking, according to docs.
And unless I am wrong, a render-blocking resource is a portion of code in website files, usually CSS and JavaScript, that prevent a web page from loading quickly.
So I am not sure it does what you want.
Also, this is intended for situations where the TemplateStyles extension cannot be used. If it is enabled, it may increase the time it takes for the mobile site to render, depending on how large MediaWiki:Mobile.css is for your wiki.

But per OrangeStar above, setting

mw.loader.using('mobile.site.styles')

might be what you want. Although, the last time I tried that, it didn't work as I expected, in fact, the Mobile.css page works by itself on its own and it doesn't require setting mw.loader.using('mobile.site.styles') on Mobile.js.