Keep this in mind here.
Isabelle is cute, yes. She's as cute as Kirby, Yoshi, or any animal-type characters designed like that, and part of her design is that she's a dog, much like the one you have at home.
Now, imagine somebody drawing your dog with human body parts (you know the ones). Now, why on Earth do people want to make a dog "sexy" or "hot"? It's a dog. It's a living being that licks it's crotch and drags it's butt across the carpet. I'm not being cynical here, I love my dog. As a pet. But pets are not something people want to see sexualized.
And in-character has nothing to do with it. You could make Rule34 of Mario coming over to Peach's Castle to "fix some pipes" and it would be in character. Does that mean it's right to show puppy p*ssy on the internet for a small minority of people to find attractive? No.
I can understand it with human characters, like Peach, Zelda, Rosalina, Samus, Palutena, those kinds of people. But why animals like Isabelle and Dixie Kong? I mean, I know that Nintendo oversexualized some of their characters (Candy Kong, Krystal, Barbara the Bat) to look human-esque. But these are animals. Not human-shaped animals, normal animals. It's just disturbing to me.
Also, the Metroid is added to the Graveyard so I'm technically on-topic still.