Here is a nice post from another forum, this is pretty much my opinion only expressed better:
Characters, however, aren't robots. They're characters. This chapter was showing what happened between Chopper and Mocha, expansion is just as important as development. (Unless you think it's a good thing to have unanswered questions to speculate about by the end of the series?)
So, when this happens, things need to happen. For example, when Chopper gives Mocha the bad news - she has to react. Furthermore, because she's a robot and she's also reacting, we should also touch upon her relationship with the other children. Sure, you could hand wave this with "they're kids, duh, of course they're friends" but that's not expanding on a character to help develop them(as this did, with the relationship between the children.)
It's not strictly about reiterating information we know, it's about expanding upon it. If you and I were close friends, and I said my parents were dead, the only reason that'd be fine with you to leave it at that is out of courtesy, but intrigue and curiosity is a part of human nature(it's how all of this progression of society and humanity happened)
So as a writer, a good writer, you expand upon things, you establish things and you develop things in relation to characters. If you feel the chapter is bad it's not because it's bad - you just don't care about the character/characters being expanded/established/developed.
All in all, the chapter was great(I totally cried), the issue is merely with your personal interest. From an objective writing perspective, as well, the fact that he expanded upon a character that is important for this (part) of the arc is good writing - which makes the chapter good.