The reception/sales of a character's game has very little to do with their moveset potential, which is what should really be given the most attention when considering characters. I've been seeing this argument a lot lately, especially when it comes to relevant first-parties like Rex, SwSh Pokemon, and Paper Mario.
Just because a ton of people think a game sucks doesn't mean the character themselves suck. They could actually have a ton of potential for becoming a very unique character. Likewise, a character doesn't deserve to get in Smash just because their game is popular or critically acclaimed. Their game could sell several million copies, but that by itself doesn't magically give them enough material for a moveset people would okay spending money on.
Honestly, it just feels like gatekeeping; someone shooting down the character without even trying to consider what they could bring to the table.