not entirely sure what you mean by "casting horror show" but although he certainly wasn't ugly, I think it could be very likely that he wasn't successful with girls at that age.
Firstly, 13/14 girls are usually more mature, both emotionally and physically, than boys (which is why they often date older boys). Even amongst 13/14 year olds boys there's a huge difference depending on when puberty hits - others in his class would be 6 foot tall and shaving already whereas Jamie is very small and slight and facially young looking. It's believable that a girl his age would look at him as a little boy (both physically and emotionally) rather than as a viable romantic partner.
Furthermore, from my experience of school, looks weren't actually the most important thing at all in whether someone was 'fanciable' - it was a mix of personality (so both boys and girls who had mixed friendship groups were far more likely to then develop romantic relationships simply because they understood how to interact with the opposite sex) and also popularity generally - looking back a lot of the unpopular children were probably more objectively good looking than the popular ones (and even if they weren't at 13/14 developed into such as adults), but it would be social suicide to date someone below you on the popularity scale.
He literally touches on this when he refers to Katie as being 'weak' after humiliated - it's like the school social caste system is basically a replica of the food chain - and then her response was in the same vein - she didn't reject him because she didn't like him or because he was ugly but specifically because, even after her popularity had been negatively affected by the photos she still considered herself to be several rungs on the 'ladder' above him.
As an older teen/adult it's very likely Jamie might have been successful with girls but I think it's very plausible that as an unpopular 13 year old he wasn't, and not having the words or experience to fully vocalise the complexities of a school hierarchy just simplified it as 'ugly.'