The problem definitely lies with Google Plus as a web app. Whatever they're doing, it's just not a very good experience on a mobile device. Maybe on Android they redirect Google+ pages to their native app, but they don't even offer a way to do that for other platforms even if you have the native app installed (even a simple link on the page that opens it in the native app would be fine). It seems obvious that they're actively trying to promote their own platform over all other reading experiences, which is a radical departure from the ideals of the web that Google has historically cared about.
This is why I think every company should hire B and even a few C+ level talent. Someone has to do the boring details.
I think this is just a common Google problem; when you hire A+
level talent they don't want to spend their time adjusting css
for every platform.
In this case, Google plus as a blog, obviously all the platforms are targets. So the CSS should be adjusted for all the platforms, and that must be part of the specs. If you have A+ talent executing, either it will be done, or they haven't implemented that yet according to their priorities, or there is something more unknown to us.
Anyway, building a G+ app doesn't exactly require an extra 30 points of IQ and 5 years of school than making a web page display readable.
Maybe they should start using CSS brain teasers in their interviews, then.