Since most (?) users use more than one platform, maybe a more reasonable question would be, what is the share of mobile reviews?
And I assume that also depends on the type of deck. In my experience, students often sit down in front of a desktop or tablet and do regular study sessions (medicine, law) whereas hobby learners (languages, trivia) tend to squeeze in quick mobile lessons whenever they have a minute to spare.
Yes. I would rather look at the priority or focus of a UI fix, independently of mobile or desktop (or both) and fix that rather than doing priorities based on platforms. Not that I would think a majority of studying is on mobile and editing content on a desktop.