I just had a look at some questions and discussions on my smartphone.
Comments on comments happen quite often (especially in the CC, where the is nothing else), but in the mobile UI those appear in a list one after another. Really important here is to see, to which comment the reply belongs, but instead you see the little arrow and then the name of the user of the current comment, not the name of the user to which comment- umm... okay. Wait.
Let's start again.
On the desktop version you have the comments threaded, so you can (kind of, depending on the level) see, which comment is a reply to another comment. The relation is visable via the hierarchy and threading.
An example (comments on an answer from the lovely mess "What do you think about our new Q&A platform?":
On the mobile version though everything is shown under eachother on the same level. The issue is, that if e.g. I sub-comment on someone else's comment, instead of a sign which comment I replied to, I see my own name as "replied to comment from".
Since I can fake it with the "desktop" version by making the window reeeally narrow, here is how those same comments look on mobile (happy scrolling):
So my expectation would be (best with link):
1. sub-comment by Eli -> comment by Veselina
sub-comment by Denis -> 1. sub-comment by Eli
2. sub-comment by Eli -> sub-comment by Denis
sub-comment by Osvaldo -> 1. sub-comment by Eli
It's not so bad when comments build up on another, but if there are several sub-comment-threads, then you are really lost on what goes where.