The idea that Mabel is being selfish in The Deep End is even more absurd.
Mabel’s whole story in that episode is about doing what’s best for the person she has feelings for even when it’s not what’s best for her. The selfish thing to do would be to not help Mermando, so that she could visit him in the community pool and be with him. Instead she helps him get back to the ocean even if it means never seeing him again.
Dipper witnessing this is important for his story as well, because up until now he’s been doing the opposite with Wendy. His interactions with her have been about trying to do what will allow him to spend more time with her, not about what she wants. About getting her away from Robbie even if she wants to date and be around him.
Then there’s Mabel’s line near the end: “don’t you know what it’s like to fall for someone? Even though you know in your heart that it’ll probably never work out, but you’d do anything for that person?”
Not only does that line draw a direct parallel between Mabel’s feelings for Mermando and Dipper’s for Wendy, in this sentence Mabel defines love not as “doing anything to be with that person” but as “doing anything for that person” even if it means not being with that person.
And this is important, because the next two episodes that deal with Dipper’s Crush on Wendy are Boyz Crazy, (where Dipper is called out and confronted with the fact that he hadn’t been considering Wendy’s feelings) and Into The Bunker (which begins with him realizing he made a mistake, and deciding to just not tell Wendy about his feelings towards her at all.)
The Deep End is not only a story about Mabel being selfless, it’s about Dipper beginning to realize that he has been selfish when it comes to Wendy.