I think there are two reasons why Maomao dislikes Rakan so much. The first is the way he treated her when he first discovered her - he went way overboard and freaked her out. And then he spent years afterwards continuing to go overboard with her, embarrassing her, annoying her, etcetera. So she had an unfavourable first impression, which was then reinforced by subsequent experience.
He's not a bad person, but he's definitely not easy to deal with - it's a running joke pretty much from this point on in the story (not a spoiler - it's pretty obvious from the get go that he's difficult). His main redeeming feature in that sense is how much he cares about Maomao, but from her perspective that just manifests as a whole lot of him getting in the way of how she wants to live her life.
He's not a bad person, but he's definitely not easy to deal with - it's a running joke pretty much from this point on in the story (not a spoiler - it's pretty obvious from the get go that he's difficult). His main redeeming feature in that sense is how much he cares about Maomao, but from her perspective that just manifests as a whole lot of him getting in the way of how she wants to live her life.