I honestly don't get why everyone is so disgusted and riled up against Shirahoshi.

I'm not a fan of crying women with huge breasts either, but that doesn't have to be all there is to her.
And why is everyone so convinced of her being weak? There is really nothing to prove that yet. Cowardice is not the same as weakness, as we all should know. It's impossible to avoid Decken's surprise attacks indefinitely. Even Whitebeard got stabbed by surprise.
I mean, if she proves to be as useless as Vivi, Conis, Caimie,... , then of course
there is no way for her to join. Usopp and Nami started from scratch too, but times are much tougher now.
But I think it could go either way at this point.
Whether she is just a stupid damsel in distress or will fight to the death herself all torn-up, using some awesome unseen skills, we'll just have to wait and see.
Anyway I think it would be good to have a powerful mermaid in the story to represent her kind and crush that pathetic racist image of weak prey and slaves. Preferably someone involved in the final destruction of Mariejois.
I'm sure Oda would have no problem deepening her character and fitting her into the crew, if he wanted to.
For example the Sunny is much much bigger than everyone thinks. And mermaids don't have to be disabled on land. Caimie just had to hide her fin on Sabaody. Shira could move like the Gorgon Sisters or use any kind of device/pet to carry her, like a weapon with Zoan DF. The possibilities are endless.
Oda can easily make it work.
Again we'll just have to be patient and see.