Craig's comment of sexless, cold and stiff perfectly summed up the routine's performance. It isn't a criticism of Alex as a person, but it is a criticism of the dance routine just performed and accurately reflected what had just been portrayed.
Could he have said it in a "nicer" manner, perhaps, but in terms of accuracy, he was totally right. If he'd said she looked as straight as a ruler, or as stiff as a board or that the routine lacked any demonstration of passion or romance he'd still have had people whinging.
On top of the previous comments, it was James who was making a big deal out of Alex's lack of sexiness (or rather her belief in the fact she can't do sexy) throughout the whole VT of their rehearsals so it makes perfect sense to criticise that aspect when such a big hooha has been made about it.
The costume she wore did her no favours at all, it showed off her lovely body but when someone is that self concious then all it's going to do is make them feel even more awkward. She would have been much better in something with a Grecian toga style which could have given her more confidence and helped to emphasis her long legs and arms.