I personally have never shipped..... like.... period.... but I admit that they are often very cute, PinkieXDash is certainly no exception......
I also would not attribute it to any one episode such as Griffon the Brush off.
I generally aspire not to analyze anything in this show and Fandom too much, shipping has a pretty thin line in my opinion for becoming cringey and unhealthy for everyone.
Again, not to say that shipping is bad or something, just moderation and all that....
As for this ship specifically well.... I love Pinkie to death, and I certainly care more for her character than I do who she is getting her love on with.... I think she has a very special relationship with Dash, and just for the record implying it's out of a mutual incompetence is simply unfair and mean. They just have a nice dynamic when they hang out together or are in a scene together, Pinkie cares deeply for ALL her friends, and Rainbow struggles with social issues often much like Pinkie sometimes does, more over, she is the only one with the energy to keep up with her.... Dash's concern for her friends is strong just like Pinkies, just shows it in her own way that people don't always understand..... Just like Pinkie.....
Compared to the rest of the Mane Six, I can't see Pinkie having this kind of Friendship with the rest of them.... I mean timid and quiet Fluttershy? Studious Twilight? Down to Earth Applejack? High class and Cultured Rarity? Out of all of them they just fit together the best, and I don't mean that necessarily in the shipping sense, but.... that certainly can be the argument any shipper makes, and it would not be unfounded. Of course, that is not to say hers or any of the friendships between them is not great or something, just different.
That is just my take.... but I don't care for shipping at all.... if it is cute and they're happy, I am happy.
P.S A cute fanfic every now and then is always adorable and fun.