>>53764590Blue was an ass you like to beat, and he popped up in the worst locations to battle you and punish or test you. Nemona only pops up before gym battles (meaning you now have to go back to the Pokemon Center and back to just do the gym), isn't hard in any sense of the word, and doesn't even compete with you. Gamefreak knew that people wouldn't necessarily like her with that whole sequence where you repeatedly say no to her.
Nemona is a terrible rival, more an annoyance than anything else.