It happens all the time and it's not acceptable.
If you comment on a status, it needs to be either:
- Related to the status itself, or
- Related to the previous comments.
If seeing the person's status reminded you of something you had been meaning to tell them, GO TO THEIR WALL and post what you needed to tell them there!
Commenting about something irrelevant kills the status! Now other people feel awkward commenting, and that's never pleasant.
Also, something that is almost equally bad: People will see a status and, in an effort to make their comment slightly relevant, say something like, "LOL, that's funny! By the way, how's your family been?" No! You just used their status as an excuse to say what you had been wanting to say.
Wall posts are a thing for a reason. Keep your comments relevant, okay?