To be fair, most blogs on WP that I visit have the 'enable mobile theme' enabled within their Appearance/Mobile options. Most themes with this setting enabled (and even some with it disabled!) ONLY show the post contents on a mobile phone and reformat the blog so that the right hand side side bar is hidden at the foot of the post rather than being visible on the right hand side. The top page tabs are then placed in a drop down menu, blog formats and images are lost etc, etc.
The new reader has not changed that so most blogs viewed on a mobile even before the new reader changes did not display correctly or as their creator intended.
I disabled the mobile version on my blog so that when people visit they get the whole blog as would be seen on a desktop PC. Clearly this is not ideal on a tiny screen however most modern mobile device allow you to tap the screen in the middle section of the blog post and they automatically expand the post to fit the screen for reading, another tap zooms back out to allow the reader to use the categories, archives and other widgets as they would on a PC.
Sadly my wife also disabled the mobile option on her blog but her theme still display the mobile version anyway and because she uses categories like me for user navigation of her site it renders the site unusable for most mobile readers.
To prove the point, my blog opens up fine in Bloglovin from the feed, taking me or any other visitor straight into the full blog whereas bloglovin still opens up my wifes blog in the mobile friendly, badly formatted and unusable version so it is nothing to do with the new reader.
The only real issue with the new reader is the extra steps required to get to that post content by loading the popup, then clicking view original content.
My theme is 'MistyLook' which with the mobile version disabled does indeed show the full post in the properly formatted blog as per a PC view. My wifes theme is 'The Yoko Theme' which still display a mobile cut down version even with mobile version disabled! It may be worth testing their theme and its settings on various mobile devices.