I think if you did parse the sentiment of citations you would get something quite bland, like 90% are very neutral (i.e. the effect of X on Y has been investigated in A, B, and C). The cynic in me would say that this is because a lot of these type of citations are a defence against getting A, B, or C as a reviewer, or even worse the citation was inserted on review to make A, B or C happy because they were the reviewer and went shamelessly fishing for citations.