I don't hate him, but at the same time, I don't get what the big deal about him is. Much like most of the Mario characters that "deserved to be in Smash more than Rosalina".
Beyond being Bowser's cute rascal son, Bowser. Jr is an interesting concept, especially for Mario, done incredibly poorly (I mean come on, his claim that Peach is his mother makes you believe that Peach ****ed Bowser for a minute - and even after you find out she isn't really his mother, it doesn't excuse the fact that
Peach had to freakin' think about it). While Bowser. Jr could have brought a lot to the table in terms of making the Mario franchise a more respectable medium for genuine storytelling (the ending of Sunshine proves that he could make Bowser more than just the big bad villain, but also let Jr himself have his own thoughts, motives and ideals), and could have had a lot of interesting interactions with Bowser, he pretty much got demoted to just being 8th Koopaling in the NSMB games, and even in Galaxy, he feels like a generic lackey to Bowser. Bowser doesn't even acknowledge him beyond Sunshine.
In contrast, Rosalina is an interesting and incredibly daring concept for a series such as Mario, done
exceptionally right. Her storybook is a pure artistic masterpiece and can only be classed as a tragedy in terms of literary genre, and everything surrounding her character is interesting and builds her personality, and she even has things to flesh her out outside of the storybook. She has limitless potential since she is a space goddess and therefore can have all kinds of powers.
I don't want this to turn into another post where I gush about her too much, and the only part that's really in response to the post I'm quoting is that I myself don't hate Bowser. Jr, but can see the many flaws he has.
But it hurts a lot when a character I know is really good despite everyone forming half-baked, negative opinions on her before her reveal, manages to still be a controversial topic even after more than proving herself worthy of Smash, inside and outside of Smash itself.