I’m ambivalent about Derpy Hooves becoming canon in TLR. Part of me thinks she should have been kept an Easter egg rather than acknowledged so bluntly, and that would have prevented the Derpygate debacle. Furthermore, I do think the name “Ditzy” may have been used, a possibility Faust suggested based on a character known by name only at the time, if only because some of the audience would object to “Derpy”. (Was I right on that one!) At the same time, that required serious boldness to pull off, and I respect the staff going that far to transcend an in-joke shared among fans and staff into an active part of the show with a real canon personality.
As far as Derpygate, I was pro-Derpy because I did not like the retconning of Derpy’s voice and appearance, but I was not as vocal about it as some and do not consider it a show- or episode-trashing issue. But that’s probably too close to typical consensus to be an unpopular opinion.
As far as fan pandering, hell, from my understanding, Adventure Time got an entire episode dedicated to an Ask blog character. I don’t consider the show “pandering” in general, although it is clearly influenced by fanon at this point.