It entirely depends on whether the subject defines themselves by their nationality. If they do then it would be fair to say that someone celebrating an English loss (for example) does dislike that person. However, most people don't define themselves purely by their nationality (which is, after all, merely an accident of birth) and so the dislike is of the flag, not the individual.
Or something like that.
If I am stood in a pub*, in England Rugby shirt, loudly proclaiming my love and support for the best country on Earth, and we lose to the Welsh,
again, then, frankly, every Celt in the pub, including the Scots and the Irish, is entitled to wade in, rip the pish and dislike me as much as they like.
If I am walking down the street, minding my own business and a complete stranger of a Celt comes up to me and says "Are you English?" and on finding that I am, launches into a tirade of abuse about how he hates all the English, and specifically, at that moment, me? Different matter entirely.
Both are real-life situations I've encountered on more than one occasion.
*or whatever the current ontrend social media version of standing in a pub is.