Yeah when players are insanely dominant, at the height of their powers, rarely lose and win most of their matches only conceding a few games, it can understandably be difficult for many people to root for them, as people especially paying spectators want to see competitive matches. When they themselves become the underdog, have to overcome adversity etc, then they often attract far more support as more people can relate to / sympathise with them. That was probably exacerbated in women's tennis when for a long time there was very little or next to no depth, the early round matches at majors were typically a joke etc.
Evert becoming the underdog to Navratilova in the 80s, or at least that rivalry becoming more competitive and Navratilova beginning to score more wins, massively increased her popularity. She was insanely popular when she broke through and did tremendously well as a teenager, but then understandably wasn't so popular (though still an incredibly big deal) during the height of her dominance during the 70s. It seemed clear that Federer was far more popular in 2016 for example than he was in 2006 when he was incredibly dominant.
In the 1990 RG final, Seles was the clear crowd favourite with Graf having won 8 out of the last 9 majors, while 2 years later Graf now as the big underdog was the clear crowd favourite. Seles to her credit and despite still being a a teenager, understood the 'bigger picture' and why fans often cheered for her opponents during matches, with them simply wanting longer contests during earlier rounds, and rooting for clear underdogs during latter rounds. Navratilova with her huge ego on the other hand failed to understand that, and often whined about fans cheering for her opponents during early round matches at majors, including when she was barely dropping games.
I've personally never seen a player 'demonised' to the extent that Seles was at Wimbledon in 1992 (when she was only 18 years old). Under those circumstances, I think she did well to reach the final. Many people including the British tabloids still hadn't forgiven her for her withdrawal the previous year (which basically over-shadowed the tournament on the women's side) and treated her as a villain, there was the 'gruntometer' saga with other players including Navratilova lining up to take aim at her, she was criticised by players like Ivanisevic for her silence re the Balkan wars etc.
But I think her 'bubbly' personality really shone through during those pre-stabbing days. Yes it's easier for a teenager that's winning nearly all the time to love tennis with their enthusiasm often waning as they get older, but I've rarely seen any player love the sport as much as Seles did. I remember her interview after the 1993 Australian Open final, when she talked about how badly she wanted the Akubra hat, there were a lot of stories about how well she treated kids working / volunteering at tournaments etc.