No, I don't think it's necessary to have a "like" button on hub comments. Personally, I think that the article itself should be the focus for visitors. Comments are usually directed to the writer of the hub who then responds to them (or not).
The comment section of a hub is not meant to be a forum for discussion, and whether or not commenters like each others comments is not important. There are forums here on Hubpages where members can discuss and express their likes, dislikes freely. I don't think that a "like" button on individual hub comments would be appropriate.