Hmmm... I tend to think it's a combination of factors. Firstly, I do think it's fairly important that the artist has an attractive style, and that the character(s) are attractive within that style, in order for them to get a large following of fanboys/fangirls.
That being said, I think it's even more important that the character is just a 3-dimensional character. Flat characters don't tend to get nearly as many fans. These characters should have all the key elements: a personality (tropes can help, but it should also be unique), desires, a goal, people they care about, good traits, but also flaws, etc.
And finally, I think the characters that really, really pull people in (whether sexually, romantically, or just in admiration), are those that seem human. This is particularly essential for villains. Honestly, a well designed villain can be waaaay better than a good hero. Have you ever noticed that some of the most beloved characters are villains with realistic motives. Like, villains can be really freaking evil, do some terrible, abominable things, kill people, lie, steal, cheat, insult your protagonists in the most disgusting of ways, but if you show their humanism, it will be hard for people not to relate to them.
The easiest trick in the book is to have a villain who loves animals, or kids, or has a family they care for. Or for their evil deeds to be revenge-based. I mean, even good heroes often have revenge as a driving force. And remember, every character should think they are right, that they are good, and that their decisions are justified. That way, even the most egregious of crimes can be overlooked (or at least understood) by the readers.