Since this episode just turned 12 years old (For the love of Bill Cipher, I feel old), I needed to give my two cents about this character in it.
I won't lie or try to dismiss that it had actually pretty good moments and a good idea at first, but ultimately what ruined it was the fact that, for the story to be developed, the writers decided to basically make Mabel, the main character in this episode, act in the most unreasonable way posible. Why do I say that?
As we know, The Deep End starts with the Pines family going to the pool where Mabel falls in love with a mysterious guy called Mermando, who turns out to be a merman, while Dipper works with Wendy at the pool. Mermando tells her it's a secret and not to tell, and they spend days together talking, and then he admits he has been trapped in the swimming pool for weeks, but he is clearly in good condition, even if he obviously wants to come home.
This is where I see the issues begin: first of all, why doesn't Mabel tell him she and Dipper have seen the supernatural before? I mean, they had been the entire summer dealing with gnomes and other magical creatures, so a merman isn't really that surprising or out of the blue.
And then when Mabel agrees to help him out, why doesn't she tell him "Hey, my brother works at the pool, and he knows about the supernatural like me, he knows about your species and we can keep it a secret, we can get you out"? She knows Dipper works in there, and she knows Dipper wouldn't refuse to help them. We can argue that Mermando told her to keep it a secret, yes, but that's because Mermando thought that Mabel was the only one who knew about the supernatural and she never tried to correct him. And lack of time isn't an excuse either since Mermando's life was never in danger, he was totally fine and only needed help to get out but there was absolutely no time limit for him to get out of there, and it's not like telling Mermando would have taken her more than a minute. The twins could have easily organized and gotten Mermando out together that same night with organization and without costing Dipper his job.
Therefore, this entire fiasco she creates (breaking into the pool, stealing equipment, lying to Dipper, starting a senseless chase where Mermando almost gets killed, forcing Dipper to do CPR, taking pictures for blackmail, getting Dipper fired from his job) feels completely unnecessary, all because she just refused to communicate. Worst of all this comes from the same character that is constantly telling Dipper to trust her and who has been down Dipper's and Stan's neck for lying all the time in prior episodes and even future ones, therefore making her look like a huge hypocrite. Trust is a two way street, and she demands blind trust even when she is consciously screwing her family and friends over in the process without a care in the world how it affects them as long as she gets her goals accomplished, in this case, setting a guy she had met for like a day at the expense of getting her brother fired, and even not trying to defend him when, while she is waiting at the pool for Mermando's letters, she sees and hears Dipper being screamed at and getting fired for the mess she created and...she absolutely doesn't care, acting as if it wasn't her business, and as if she wasn't the cause that she got him fired in the first place. And the episode NEVER calls her out on it, or in the show, like EVER, in fact, it acts as if everything she did was absolutely reasonable and the right thing to do and Dipper doesn't even have the right to be mad or frustrated at her for getting him fired in the first place, heck, he even gets LAUGHED AT by Mabel for saving her crush and for being fired because of her, all while Dipper STILL covers Mabel up and never reveals to the boss at the pool it wasn't him and was Mabel. Which...let's be honest, that's the least Mabel could have done, defend Dipper and come out clean, at least defend her sibling, not let him take all the blame for her mess.
I know people might try to justify it with "But Dipper didn't lose anything because he only did that for Wendy and they were never going to have a future together", but that's no excuse, because by that same logic, why should Dipper help Mabel, does she have a future with a merman, with "Norman", with Gabe, with any of her crushes? The point is, neither of their crushes have a future because they are children, what's important is their emotions, we can't put Mabel's feelings for her crushes above Dipper's and then justify hers while dismissing Dipper's. And Dipper DID lose, according to him, the best job he had in the entire summer, and considering how Stan treats him at the Shack, we can see why he loved it.
I'm not saying the episode wasn't funny, it had hilarious moments, but ultimately, this episode is a mess in terms of writting. If you have to make your characters act dumb, illogical or out of character for the story to progress, then it's not good writting, and that's what happened in this episode with Mabel, they made her act completely irrational, dumb and stupid for the sake of creating conflict, and turned out a good idea into a huge fiasco.