I think what comes down to the list is "Are these hobbies broad enough for both partners to enjoy together?" (biased opinion of the girl/guy being asked what he personally likes, basically). For manga, there are SO many types of them ranging from romance, to horror, ones for kids, ones for adults, etc. It seems almost impossible for both people in the relationship to be unable to find a comic they can relate to.
For the ones lower on the list, I think dream girls would not be tolerated by girls, because that would be their boyfriend's waifu and the guy might seem too otaku-ey and have tons of figures of 2D girls lining their room. Guys might not appreciate their girlfriend reading BL because they probably don't read any.
For Vocaloid not being very tolerated, I think most Japanese (including otaku) don't know ALL the Vocaloids out there and probably only know the main Cryptonloids and a few others. So maybe they only know a handful of popular songs (like how "Senbonzakura" was on Kouhaku Uta Gassen) and don't care about those songs/characters, even though they actually have tons of characters/songs to choose from without knowing. I guess we have to keep in mind that Japan is probably aware of (at minimum) Miku's activities such as Snow Miku, maybe they think it's annoying/weird? (Kind of like how Miku is a joke to most English speakers who don't know about the software/scene.) Also, the VocaVerse circle (including our Twitter bubble) are full of hard-core fans, think of the people answering the poll as casual normies.
Lastly, I've read a lot of really stupid dating polls in Japanese, so don't get too insulted by this one. I saw this video recently where ideal boyfriend requirements were REALLY dumb (like he should work part time at a cafe, wears rolled up sleeves, likes cola):
A couple years ago, magazines were even talking about guys being carnivores vs herbivores (basically being an alpha male vs a beta male in the dating world), I feel like getting love advice from a country with a rapidly declining birth rate/people choosing work/hobbies over dating is probably not the best thing to lose sleep over.