I can understand the one-sidedness with regards to Marina's popularity, especially in the west. Marina seems more mature, she's an octoling, and she's "poc"-- shes a lot of new things for a Splatoon mascot all in one.
Pearl seems like obvious loli-bait, not very creative and a difficult personality to enjoy unless you are a fan of bratty lolis. Which is alright I mean I'm not here to criticize anybody's personal preferences but there is something to be said about designing a character for a grabby waifu fanbase who looks and acts like a spoiled 10 year old trying too hard to be trendy.