The idea of Vampire Jubilee being poorly written spawns not from her being a Vampire, or even being not-mutant, but rather from her behaving not-Jubilee. It was happening ever since her depowering, but the vampire change really cast it to the forefront and drew attention to the personality change. Jubilee was a spunky, upbeat, positive teen hero through all sorts of terrible trials. Look at how her early appearances went, saving Wolverine from his crucifiction at the hands of the Mauraders who were murdering mutants in the most terrible, violent manner. And how does she respond to witnessing these horrible, traumatic experiences? Totally a glass-half-full girl.
Over the years, Jubilee has been beaten back, demoted, watched friends die, and still she managed to keep that positivity. That was a vital part of her character. The fact that she was the light-hearted kid at heart was what allowed her to balance out the grim cynicism of Wolverine. Now this is girl who can watch some of her closest friends be murdered right in front of her and still bounce back to make the most out of life, and we're expected to believe now that she is so self-absorbed that being depowered spiralled her into this deep, dark depression, and becoming a vampire turned her into this grim and brooding creature. This isn't just character development or natural progression; the mangling of her personality has been forced, feels entirely unnatural, and as a result it is not at all being well-recieved.
Vampire Jubilee is generally disliked for not being like Jubilee, not for being a Vampire.