He's probably pretty cute, just had a crippling social condition prior to the events of the manga. Girls like Yuno who are considered cute and smart by everybody else, they expect everything to just come to them. And you know how girls want things of value they can't have? So when he said something that unintentionally impressed her, but didn't show any overt interest in her, it was over for him. Because she's obviously had some strange sort of upbringing that left some traumatic seed in her psyche, the repression of which is probably the reason she was such a model student. With that little shove over the edge, a stalker was awakened.