I'm pretty sure Simon is the most popular Belmont.People confuse "popularity" with "requested". Jusy because a character doesn't have a vocal fanbase saying "add X character please" doesn't mean they aren't popular.
Ken is the second character from the biggest fighting game series in existence.
Richter is easily the most popular Belmont.
Any Pokemon starter is popular unless they're Meganium.
These characters may not have been talked about within the dedicated Smash communities but they certainly had a lot of fans before their reveals.