While Dash's actions weren't appropriate, she DID ask the sensible question: "What makes you so special?" as Trixie's act wasn't living up to her boasting, blatant false advertising. Effectively, she's a charlatan.
Also, Dash stopped causing a scene once Trixie made the Ursa claim, but it was Trixie who started making random challenges and taunting the audience. In fact, the first one she tried to pick a fight with was TWILIGHT, who even Trixie's defenders admit hadn't done anything wrong.
No "heckling" went on, Rarity made two irritated comments directly in response to Trixie's bragging, and a third comment explaining to Twilight why they were annoyed, and Applejack made ONE comment that was nothing more then an explanation to Twilight as to why they were annoyed
Not to mention Applejack only stepped in because Trixie tried to pick a fight with Twilight.
To top it off, Trixie goaded Rarity into a confrontation, not the other way around. Heck, Rarity was criticizing Applejack and Rainbow Dash at the time, not Trixie. You'd think Trixie would appreciate the support
Trixie did NOT fight the Ursa to protect the town, she did it to maintain her lie, as evidenced by her only stepping in when Snips and Snails reminded her of said lie.
Applejack, Rarity, and Dash did not "mock" Trixie, they were explaining to Twilight why they don't consider her a showoff like they do Trixie
You called those three "jackasses", but you're ignoring that Trixie was condescending to her customers, rude to her fans, and downright malicious toward those who called her out on being a charlatan, not to mention she talked down to Twilight after Twilight's actions effectively saved Trixie's life.
Two of Rarity's comments were DIRECTLY IN RESPONSE to Trixie's bragging, her third comment and Applejack's ONLY comment were explanations to Twilight, and both of them spoke in a normal tone of voice, with neither of them making any attempt to actually cause a scene or embarrass the performer, as per the definition of heckling.
Dash's comment was inappropriate, yes, but she HAD REASON for what she did, and Trixie certainly made no attempt to take the high ground. Once she made her Ursa claim, she had dealt with her lone "heckler" who was no longer causing a scene. Trixie could have left it at that, as the audience was suitably impressed, but instead she chose to pick fights with random ponies, no even with her alleged "heckler". Calling out a charlatan for scamming their audience =/= heckling.
You make it sound like Trixie stood up to the Ursa right away. She didn't. The first thing she did was RUN AWAY so she's not as heroic as you make her out to be. Not that I blame her for not wanting to fight the Ursa, but the fact remains that she and her "hecklers" had the same initial reaction. She only fought the Ursa because her pride wouldn't let her admit to being a fraud. As shown when Snips and Snails reminded her of her claim. It was egotism, not courage that caused her to face the Ursa. Pride causes people to do stupid things, look at Rainbow Dash.
Applejack's comment "All hat and no cattle" meant that Trixie can't back up her bragging, which is true Applejack never claimed to be able to defeat an Ursa. How were they being Jackasses? Dash called Trixie a loudmouth, which she is: "a loud and talkative person, especially a gossip or braggart". Rarity said that she's "most unpleasant", which she is, see below. It was Karma, Trixie brought that on herself with her own behavior by being a jackass herself.
No, Trixie was condescending to her entire audience: "Anything you can do I can do better" she says to the entire crowd, not just those three. An entire crowd of ponies she's never even met, yet she claims to be better then them at the things that define their existence. You're also ignoring that she was rude to Snips and Snails, who are her fans, twice tried to humiliate Twilight, who had done nothing wrong, and then talked down to Twilight after Twilight essentially saved her life. On the flipside, Trixie was NEVER nice to anyone.
About "no one else taking issue". I can't help but think that's because they're the main characters, they're naturally going to be the ones delivering the message, and of course, receiving the focus. So of course the writers aren't going to show the negative reaction of anyone else, they're not the focus. The writers wanted to deliver an Aesop about bragging, and they needed the Mane Characters to be the one delivering the message.
The moral of the episode is: "Don't be a braggart, lest people call you out on it, or you get trapped in your lies", both happened, and it's to be expected when you annoy others by talking about yourself so excessively like Trixie did.