I think MM is fairly articulate (in terms of presentation if not content), fairly bright, very driven,and very attractive. That's a combination which can take you far in life. And then if you a marry a prince, it's like jackpot!! And that combination in the royal family could have made them real assets, for the 'good' they want to do.
But, she/they have mistaken this for some inherent amazingness about themselves as a couple.
Their downfall is their ego.
Harry is impressed by her looks, drive (to 'do good' as he's dim enough to think this is a goal in itself) and her articulate ability, as he will be aware that he's punching above his weight in all those categories. So he's swept along in the narrative she's driving. Not because she's 'vile' but because she's the brighter of the two. It also fits with his persecution beliefs around the media, created from his mothers death.
But she's not that bright, or really articulate in terms of content, or a good strategist, as it's all a disaster for them (with the British Public at least) and such a missed opportunity for the roles they could have created within the establishment if they's given it time and effort.