Yes, the placement of the main content (MC) in the page layout is a ranking factor, according to an official statement from Google. This signal is tied to the “page layout update”, and it can be either a positive or negative factor, depending on where the main content is placed. If the main content is displayed above the fold, it’s been given the full SEO weight. If it’s buried down thousands of pixels (i.e. by ads), the content is weighted less:

“If you click on a website and the part of the website you see first either doesn’t have a lot of visible content above-the-fold or dedicates a large fraction of the site’s initial screen real estate to ads, that’s not a very good user experience. Such sites may not rank as highly going forward.” source

The position of the main content on mobile UI also seems to matter. Gary’s statement suggested at the 2018 PubCon that content position on the page, on mobile, matters:

“ordering of content on mobile may impact how you rank”. ”when you move something up higher on the page, it would be better for that page appearing in search results.” source

However, John tweeted that,

“if it’s useful content, we’ll treat it as such regardless where on a page it is”. Maybe we has referring to desktop only? Not sure… source