I think you need to rewatch all the episodes he's in. He wasn't just a jerk in Putting Your Hoof Down for the sake of plot convenience, he was a jerk because that's his character. His only appearance where he wasn't a jerk, a grump, inconsiderate or a crybaby was Hurricane Fluttershy.
I'm definitely thinking you need to as well. He's stern, not a jerk. In Ticket Master, he helps Fluttershy in trying to get Twilight to give her the extra ticket, convincing her to keep going when nervous. He shows Fluttershy the smoke coming from the mountain in Dragonshy. He convinces Fluttershy to go to Celestia's welcome party. He refuses to let the CMC see Fluttershy after they write gossip about her in the newspapers, then allows them to after their apology article. In Keep Calm and Flutter On, he warns the group about the flooding at Sweet Apple Acres. Once Spike treats Angel better in Just For Sidekicks, he saves him from getting noticed by retrieving the lost gem and feeding it to him. He encourages Fluttershy to sing in Filli Vanilli. Just where is this jerkish, grumpy, inconsiderate Angel you speak of?
He has his moments, but it's his overall demeanor is what matters. Yes he (reluctantly) helps Fluttershy in trying to get Twilight to give her the extra ticket and boy does he look happy about it. He does show her the smoke in Dragonshy, but not to be nice. It's after he refuses to eat the rest of the carrot he was happily munching on - for being warned not to eat too fast - and runs off that he breathes in smoke, which makes him cough. After that it's just him trying to get Fluttershy to understand what he's saying. When she finally gets it, he throws a carrot against her head, since that's what good friends do, I guess. I'm gonna be nice and give that a pass. In the same episode Spike is assigned to watch over him and the other pets, which is a job Spike takes seriously in this episode, and right off the bat Angel causes mayhem from that moment until the end of the episode for no reason at all. He hops onto Spike's head and stomps, because potatos. Nope, not a jerk move at all. Angel convinces Fluttershy to go to Celestia's welcome party, but, again, it isn't clear whether or not he cares. The scene is ambiguous and could be very well interpreted as him just wanting Fluttershy to be out of the house. In Putting Your Hoof Down he's downright a prick. There is writing a character out of character for the sake of plot convenience. But that wasn't what happened here. That happened in Spike At Your Service. Angels behavior in this episode does not contradict any of the spoiled, bratty behavior he displays in other episodes. Here, we just see the full extent of it when he isn't reeled in. In Hurricane Fluttershy he is sympathetic and supportive of Fluttershy, just like in Ponyville Confidential. Because caring for Fluttershy in her times of need is his other definitive character trait. In Keep Calm and Flutter On, Angel does warn the others about the flooding. In the same episode he makes a fuss about giving up his favorite spot on the couch only after he's asked if he's okay with it. This doesn't make him a jerk, but it is another callback to how spoiled and domineering he is. In Just For Sidekicks, Angel was causing trouble throughout the entire episode. Even before Spike was petsitting he was giving him a hard time. Later on, when Spike actually starts petsitting, he seems determined to give Spike as hard a time as possible. Why, we don't know. In the beginning of the episode we were shown that even though Spike was only in it for the gems, he was actually going to do his job, until Angel decided to run off and he had to be pursued. Only as the episode progressed did Spike become less responsible and did Angel's actions become justified. Still, that wasn't the case before he ran off. Angel was hell-bent on making a mess from the start no matter the circumstances, just like in Dragonshy. Even the other pets were shown to dislike him. In Castle-Mania, Angel sets off a trap in which Rarity and Fluttershy get caught. Even though this was accidental, how did he deal with it? He chills with Twilight and Spike eating all the carrots he could want, not giving two cents about what happened. For every instance where Angel is sympathetic, there is at least one other instance where he is being selfish. This defines a jerkish character.
No, it defines he's spoiled, and yes, he definitely is. But he's cared for by Fluttershy, and we know she spoils everyone no matter what, just look at Keep Calm and Flutter On & It Ain't Easy Being Breezies. He's only spoiled because that's what he grew up to be under Fluttershy. It's really not his fault. You can say he just wants Fluttershy gone, but it's clear he's just trying to help her stand up for herself and protect her and trying to get her to do what she wants when she's too afraid to. Yes, throwing the carrot at her was overthetop, but he was trying to get her attention when she wouldn't listen. He had every right to be upset over losing his seat in Keep Calm and Flutter On. It was Discord, after all. Plus, Discord pretty much pushed him around in the episode. Is be upset and rude, too. He wasn't hell-bent on making a mess in Just For Sidekicks, he was hell-bent on getting to Fluttershy, because he didn't like being cared for by ponies other than Fluttershy, especially when Spike didn't care for doing the one thing he'd wanted throughout the whole episode. By Castle Mane-ia, he's more adjusted to the others, because Spike actually shows care for him by the time he admits he's wrong and fluffs his tail, which was all Angel really wanted in the first place. Yes, not searching for Fluttershy was wrong, but Rarity was with her at the time, so maybe he just knew she'd be fine? Besides, when he saw Fluttershy get hurt near the end of the episode, he showed genuine concern for her. And no, he didn't try to stop her from forsaking out at the end, but Twilight and Spike didn't for a bit until it got really out of hand.
But I do agree, Spike was really out of character in Spike At Your Service.