If you want more skins for an unpopular champ, how about you start by actually PLAYING that champ?
Now before anyone screams bias, yes I play popular champions. I play Ahri, Ezreal, TF, Corki, Lux, Orianna, Lucian, Vlad, etc. All of which have a variety of skins. So yeah, I do enjoy that.
Do you wanna know why they get skins? Because they're fun to play. Because they have consistently dedicated fanbases who will play the champions and they're enjoyable enough that even when they're bad they get good play time and when they're good they're really popular.
So those are good targets for skins, because they make Riot the greatest net revenue. Sometimes, if they get really good ideas, they'll make them for the champion they think it fits best, but usually they do skin lineups.
You wanna know who doesn't get picked for skin lineups? Unpopular champions. You know what a good solution would be? Playing those champions and bolstering their playrates. If you actually enjoy the champion, them having/not having a skin shouldn't hold you back or perturb you in the first place.
I have a friend on my collegiate team who has been a dedicated Urgot one-trick for nearly 2 years and he will never get a skin for Urgot. This is because he actually enjoys playing the champion but everyone else who is crying for him to get a skin is sitting on the sidelines and not actually enjoying the character. Riot won't give a champion a skin if nobody plays them since that's just a waste of resources.
So if you actually want a champion to get a skin, play the damned champion. Main them. One trick them. I don't care just stop bitching about them not getting play time and attention when you refuse to play them.
As a footnote, every "Yorick needs a skin" thread I go into, I op.gg the player to see if they actually play Yorick. It might come as a surprise but they rarely do. If you come into this thread justifying why Yorick, Cassiopeia, or Illaoi hasn't had a skin in a long ass time I'm going to op.gg you and I wanna see at least 3 games with that champion.