Well, I guess I should give out my two cents about this situation...
First of all, I'm completely neutral with both girls. Same for the Squid Sisters, really.
It's interesting to see fans trying to justify Pearl's presence in this band, in every possible way... while forgetting the true reason why she wasn't received well in the first place: Pearl didn't grow up much... only her forehead, which I don't really notice as much. That's the whole reason why this situation even exists.
I understand that making a "diverse" team is actually very Balanced, since the team isn't really the worst in many aspects... it's a solid group! Unfortunately, this type of style has its own weaknesses... it doesn't really have any definitive role! People do wanted to see the team going more in the direction where Marina goes... not just because she looks cute or she is an Octoling, but because she also appears like an adult!
Splatoon 2 does have a lot of elements for kids, but many seasoned players are adults and wouldn't mind at all if this series grows up in at least certain aspects of it... and Marina did bring up the possibility that the new idols were finally appealing to the more adult audience! But then you have Pearl, which looks like that bratty sister who loves to tease you and be generally very childish and more in the way...
That must be the reason why Pearl is so hated! It's because people feel that she gets in the way for having a band who looks much more mature than what we had before with the Squid Sisters! Even her voice sounds very cutesy, which makes you wanna say "Aaaaawww, isn't she an adorable little girl?!".
The best solution for this would be to see Pearl acting much more like an adult would... but you see her having this "bratty" attitude even outside her performances with the gestures and you hear her rap-type dialogue, which feels like a little girl who hates to have some kind of good manners! Even during the Splatfest announcement, what she has nearby? Just a chair to sit on, like a little girl who watches a mature woman making all the music work.
Again, I don't hate her presence... it does give a very Balanced band, which is at least much better than having a totally kiddy band as the new idols! But it doesn't always work... and sometimes, you should get down a more definitive, precise role. Just like with certain weapon which are much better on certain maps, certain band formations should be more effective rather than following a style this much.
That said, we will see if Pearl will gain more popularity over time... after all, she fell straight in the "underdog" category judging by the whole audience reaction about it. Underdog-type of characters are known to have a huge fanbase behind them, after all... yeah, we will see.