It really ought to be annoying, and yet it's charming.
"A Friend in Deed" is an episode that shows Pinkie at her most irritating, then proceeds to make her actions fun to watch. I'm not entirely sure how the show did it, but through some miraculous combination of high energy and character development it works... most of the time, at least.
Pinkie, at least as shown in this episode, is too adorable to hate, and just too good-natured to really hold anything against. Her intentions are mostly good, though they do seem a bit self-centred. Pinkie's interested in making others smile and making friends, though this episode does show how that could possibly be a bad thing. It's an interesting twist on Pinkie's usual attitude, and while it was explored before in "The Best Night Ever", "A Friend in Deed" goes farther with it.
Cranky provides a good foil for Pinkie, making Pinkie's already enjoyably silly antics all the funnier due to the great comic pairing of the two. There's a number of laugh-out-loud moments, but as they stem from Pinkie relentlessly bothering Cranky, they could easily be found irritating by some. Personally, I found myself entertained, though the episode often came perilously close to just being annoying.
However, it's the character depth of seeing the darker side of Pinkie's relentless friendliness and having her come to terms with its consequences that makes this episode interesting. Pinkie learns here that some people don't want to be instant friends, and that she can't make everyone smile. I think there's still a great potential episode to be made from this, but what we have here is strong enough.
Finally, the episode is packed full of songs, but most of them miss the mark, mostly due to being more silly musical sketches than proper songs. Like S1, Pinkie often takes a common domain tune and puts different words to it, which makes sense in character but doesn't make for particularly engaging listening. Of course, when Daniel Ingram commits to a more unique tune, like the "Welcome to Ponyville" song, then we get a much more fun musical number. However, the real show-stopper is the "Smile song", by far one of the best songs the show has ever produced. It oozes cheer, and is bursting at the seams with pure, concentrated joy. It shows off Pinkie's main goals in life, more or less summarizing her character's simpler elements. It also helps that the tune itself is a gloriously upbeat mixture of strings and synths, and that the chorus is one of the show's catchiest, and that the verses are very well written. It's just such an excellent musical number.
"A Friend in Deed" is a very fun episode with a lot going for it. There's plenty of fun to be had here, though some may find it more annoying than anything. Sure, it's not the best episode, but it's one that I find enjoyable, and which has a lot going for it, boasting some strong character development and writing. It's a good episode.