1 - How would you change the comment sort other than reversing the created at order? In that case you need to make create.js.erb aware of the sort order and use append instead of prepend.
2 - Apply the 'admin' class for admins, but leave the even-odd striping to CSS. Something like this ought to do it:
.comment:nth-child(2n) { background: #eeeeee; }
Also it is more efficient to build your node and manipulate its attribute before you insert it into the DOM :
var comment = $("<%= j render @comment %>").addClass('comment')
comment.addClass(new_class)
$('#comments_div').prepend(comment)