She's a popular character whether people want to admit it or not.
Its funny Lightning lovers want to trot this shit on every thread, when its really not.
Most people dont like her or are indifferent to her. The declining sales are there to corroborate it, and she is not talked about much as a character in forums or even real life talk, to someone not shit on her character.
A popular character that is still hated by some are characters like Rosalina from Mario, Elsa from Frozen (funny as this two use nearly exactkly the same design) or Isabelle from Animal Crossing. Just to name a few.
They have design and characteristics that attract lots of people (males and females) from all ages, even if theres still people that hate them because of that same tropes and design elements they are using. But those characters sell, and the two companies (Nintendo and Disney, in this case) know it and merchandise them appropiately.
Lightning has neither of those, even if Toriyama tried time and time again to push it like she did. She has no selling momentum, and the tropes and designs she uses are only liked by a few number of people, even if they like to be vocal about it (theres basically no market for her outside of a small niche, and I can assure you thats not because she is an "anime" character, but just becuase she uses tropes of anime very few people like).
Cloud had the luck of being in the right moment at the right time, and in a good game. If not he would probably be hated similarly if he appered for the first time in similar Finals as XIII right now.