This is like asking.... Why does Len always die? Why is Rin always bossy? Why is Miku always sweet?
Going off of what zerowakakkoi said, and especially since you're talking about fanfictions and not songs; basically the 人気 headcanon of Meiko is that she's older than most of the other Vocaloids, drinks, and isn't totally a nice person.
Also, Meiko, while a lot of people know her; she's not as 人気 when compared to the other well known ボーカロイド (Miku, Rin, Len, etc.) so people are probably also もっと見る willing to make her the villain than other ボーカロイド that they like, that's kinda my own personal speculation.
Long story short, don't take peoples hate コメント against Meiko personal, it's just like あなた say; it isn't true. Crypton has not 与えられた any of their ボーカロイド ANY canon personalities, traits, items, etc. there are just things, like Meiko's sake, that are widely accepted headcannons.
I don't really know since she is very motherly and kind , but probably just some idiot haters または Mikutard . I guess she is being the bad guy in KaiMi ファン fiction right ? . but she wasn't always being a bad guy , for example read some KaiMei fanfic.
They're n00bs. They're going to put a lot of clichés in there, especially Meiko being the bad guy and Miku being the good girl. If あなた headcanon the opposite, then that's okay. 公開する it on the web if あなた want to.