Lately, when Discord is at his best, he's a jerk with a heart of gold. Without his inherent edge, he's a completely different character in his voice. Discordant Harmony and especially The Break Up Break Down handle that side very effectively. But when he's just a jerk, he loses that dimension and becomes antagonistic just because. And that's the case here. Because he's not leading the school, he makes Starlight, established previously and here as a friend, miserable.
His worst moment, very clearly, occurs at the buckball pitch. His scheme with the bugbear put the Student Six and a few others in really grave danger. Had Starlight not scare him off, Yona would've been stung. Recall how Dash's and AJ's selfishness and recklessness nearly caused her to drown? This is no better. Had she be hurt on Twilight's grounds, then the school would be in big trouble. Starlight was absolutely justified to be angry at him, warn him, and blast him away from the school grounds after he continued his defiance. Back to him later.
Starlight and Spike continue their successful roles since Season 7. AMoP is the first episode since The Crystalling to pair them up, and you immediately see their trust and faith for one another after Twilight temporarily promotes her. He's always by her side and helped put in the work to alleviate the stress. When Discord and Starlight didn't see eye to eye, Spike was the middle man to maintain order between them and be as objective as possible.
Was Starlight justified to warn and blast him away from the school grounds? Absolutely. As headmare, it was her job to punish him. But Discord isn't like other beings; he's a god with a love to warp logic. Rather than talking to him what's wrong, Starlight chose a very drastic measure, which Spike rightfully warned would only worsen matters. Why? Because provoking him proved him right, even though he was insufferable. Therefore, he felt justified to raise more hell in the School of Friendship.
Discord has insecurities and sometimes fails to hide them. But the clues, even if subtle, are missing, because that context when Starlight spoke the episode's primary lesson is missing, too. Everywhere he showed up, Discord becomes a destructive brat, and acting like he doesn't know either her or him hurts, too. Was she right to apologize to him? Yes. But by offering him a job, the story lets him off without any level of consequence and rewards him for it. The RM6 return, nullifying the offer, but it's still an unsatisfactory resolution.
On top of that, almost all the jokes fall flat. The ones with Twilight fail, because she's flanderized: Her personality early was reduced to obsessively organizing and pre-planning everything to the point of being redundant and at Starlight's expense. Cranky constantly spitting his drinks (accidentally) at Gallus's direction while being a lazy ass regresses him. One of the only jokes to be funny is Trixie talking on her banana phone.
Despite being rather negative (and not liking it), I won't throw in reactionary hyperbole and call A Matter of Principals bad, awful, or an atrocity, because it's not. At the time of this quickieview, it's the third-worst episode of S8, but nowhere close to the badness of Non-Complete Clause and Fake It; it's watchable mediocrity.