In a Google-Sheet called Field Schedules for Google web-apps at http://drive.google.com you can right click on a cell to open a context menu with the option to "Insert comment." Let's say for example I insert a comment: "hello world." into the cell "A1."
After I create comments, I attempt to open the same Google-Sheet, Field Schedules on my Boss's iPad, or on my LG-G2-Android device and I do not see any option to "Insert comment[s]." Nor can I find comments that already exist.
After exiting the Field Schedules from the Android & Apple devices I can confirm that the original comment exists by using my desktop computer to open Field Schedules and hovering my cursor over "A1" it will then display my comment "hello world."
Best Answer
The answer would appear to be: You can't. (At least right now.)
From Google Support, regarding the Android app:
Google Support, iOS app:
Collaborate on documents in a mobile browser doesn't mention comments at all. However, from my experimenting, it doesn't look like comments are available there either. (Which makes sense; the mobile browser version is a limited subset of functionality.)