My problem with the episode other than diamond tiara getting redemption way too easily is that the cmc didn’t earn their cutie marks and have shown no proficiency in helping prior to the episode. In the five seasons that have been on, the only time they haven’t helped anyone without being the reason they need help in the first place was that one unlucky guy.
What they have done is almost kill babs, nearly ruin a big opportunity for rarity, destroy a talent show, cause a plague, lie to children and nearly embarrass their entire town during the Olympics.
But they did earn their marks. They have been practicing their talent since their introduction. It isn’t about just helping others, but about helping others understand themselves, and that’s what the CMC have always been about. Their efforts had always been misguided and turned inwards, but what they did to earn their marks is what they have been trying to do since the beginning.