I think, so far, Althea fits into Mabel's backstory in a way that is fine--it doesn't retcon anything and we hadn't heard about any friends in Bayport before--but also doesn't add anything to Mabel's character.
The stated reason Althea is there is to explore why Mabel has a hard time trusting people (or rather says she does; the evidence isn't there but that's a post for another time.) But that wasn't remotely a mystery. All at once her best friend died, her other best friend went to prison for it, and her third best friend saw what really happened but wouldn't say anything about it for reasons that were indecipherable to her. Even before that, her dad was dying of cancer and her parents lied to her about it. We do not need another backstabbing friend to explain why Mabel struggles to make friends her own age.
Maybe it'll add something to her character by the season's end, but I'm not optimistic on that front.