I always enjoy it when Starlight Glimmer's a little snarky, self-centered or ill-tempered because those are the times in which Starlight feels like a villain who's trying to change her ways. I think this is why On The Road to Friendship was such an enjoyable episode: Starlight feels like she's a villain who's trying to change her ways, but still hasn't shaken all her old habits. She's inconsiderate (bringing along three steamer trunks of stuff even when there's clearly no room for them, buying food without Trixie's permission, eating the last haycake and selling Trixie's old caravan), sarcastic and generally a pain in the flank, but you also know she's genuinely friends with Trixie. One reason I enjoy episodes that focus on Trixie and Starlight is that the two of them have a chemistry that's completely different than that of the Mane Six: they'll bicker and insult each other and their egos will clash. You can tell these are two ponies who are still new to this "friendship" thing, making their reconciliation at the end all the more satisfying.
As an aside, I liked when Trixie and Starlight made jabs at Twilight Sparkle. It always seemed odd Starlight harbored no animosity towards Twilight, despite having been mortal enemies with her a season prior to her reformation. I always believed Twilight and Starlight's relationship should've been like Goku and Vegeta's in Dragonball, with Starlight initially being resentful of Twilight and only going along with her friendship lessons because she had no other option, but eventually growing to grudgingly respect Twilight while still throwing the occasional jab at her.