I'm going to disagree with you here. The show has a limited casting budget, so they couldn't afford to hire many actors for Meereenese characters. We really just have Hizdahr (plus Mossador as a promoted extra, but he's not around that long and wasn't originally planned to have much of a role). As such, Hizdahr's show version does a lot of the work of covering elements of the Shavepate, the Green Grace, etc. His character has to be written in a way that, like with the ensemble of Meereenese characters in the book, Dany is shown to be out of her depth in a city she doesn't really understand, working with people she doesn't want to work with.
They do this by making Hizdahr a more earnest, reasonable character, who is fundamentally prepared to work with Dany in an honest fashion, without an agenda of personal profit. When he first appears before Dany, as a loving son asking to bury his father, it's clear that he doesn't fit into her mental template of the evil slaver. Even though she had his father killed horribly, he kneels before her and makes his request respectfully. Struggling with her conscience, she allows him to bury his father and appoints him as her ambassador to Yunkai.
However, Dany draws the line at accepting any suggestions of his that don't fit her idea of how society in Slavers' Bay should be, such as the one condition that the fighting pits could reopen (to free combatants only). Since she has dragons and soldiers, she insists that the people of Slavers Bay must accept her will or die. Thus Hizdahr appears as the voice of politics, consensus, and compromise, whereas Dany appears as a despot. Her despotism seems to have the value of personal freedom behind it, so we tend to sympathise with her, but it's despotism nonetheless. People have to be free in the way she wants them to be, as we see when she outright refuses Hizdahr's suggestion that she meet with free pit fighters and listen to their views on the reopening of the pits.
Hizdahr fairly consistently gives Dany sensible advice. He explains why he doesn't think that the Sons of the Harpy are backed by the noble houses, and the attack seen this week at Daznak's implies that he's right. (I say this because the Sons seem to attack plenty of people wearing noble costume, and of course even murder Hizdahr himself.) He also says that, if she had to execute Mossador, she should have done it quietly in the pyramid rather than making a public spectacle. The riot that breaks out afterwards backs him up. By giving good advice, he is shown to be competent.
After Barristan's death, Dany arrests him and the other heads of the great houses, without any investigation. She has one of them burned alive and torn apart, without any idea if he is guilty or innocent. If she tried to investigate, she might've learned more about the various house leaders, but she doesn't seem interested in learning about them. No: they must learn from her. I don't think this makes Dany look very kind or fair; I'd say she looked unsympathetic. For his part, Hizdahr shows some bravery when confronted by a messy death (a sympathetic quality), though he later shows his fear in private (which, in my view, evokes another kind of sympathy).
Although Dany then agrees to open the fighting pits, her proposal that only free men fight is no different from what Hizdahr already suggested when he first brought the proposed concession to her. She's gained nothing for herself by delaying. And, as for the marriage, she doesn't ask Hizdahr if he is willing to marry her, but merely orders it. Even when she's supposedly making concessions, she's actually dictating terms.
Even though Dany agrees that Hizdahr was right about the need to forge bonds with the people of Meereen, the way she treats him as her betrothed doesn't show much respect for the city she's conquered. She makes a bit of an effort by attending a local pit fight, but that doesn't last long. Her public lack of regard for Hizdahr reaches the ridiculous point, in this episode, of letting her lover lean in between them at a massive public event and brandish a weapon at Hizdahr's neck. All the while, she's visibly enjoying his discomfort. And, when the Sons of the Harpy attack, in spite of everything she's done to make Hizdahr's life worse, his last act is an effort to help her escape from the arena. Dany, for her part, doesn't even seem to notice or care what he does or that he dies.
I don't think Hizdahr comes across as a moron on the show. He speaks eloquently and, for the most part, courteously. His mission to Yunkai, where the people actually listened to him, was very successful. I think that his qualities on the show illustrate that the nobles of Slavers' Bay aren't just the evil caricature that Dany imagines. Certainly, many of them are - the fact that a majority approved the crucifixion of the slave children shows that - but not all. And Dany's interactions with Hizdahr show her almost total lack of willingness to consider that possibility. She has to believe in her own values, has to see herself as the righteous saviour. Even in a situation where she has a noble who's being as cooperative as possible, she doesn't use him to anything close to his maximum potential. She just can't trust him. Consequently, he's wasted on her.
So, yes, Hizdahr's death in the show looks pointless... but I actually think that was the point. His death was a waste, a culmination of a whole series of efforts to work with Dany all that turned out to be a waste. He wasn't associated with the Sons of the Harpy: his final moments prove that beyond reasonable doubt. He sincerely tried to work with her, for the sake of peace in their lands. And Dany didn't care, because at a fundamental level she hadn't persuaded herself that people like him had value. She flirted with the idea when she chose him as her ambassador and then her betrothed, but it was nothing more than a flirtation, as she didn't take on his recommendations as ambassador until it was too late, and didn't treat him with respect as a betrothed. Her "break the wheel" speech showed a lack of regard for noble houses in Westeros as well - but, in her mind, she's the convenient exception to that rule. Her right to rule as the heir to her royal house is based on the very system that her egalitarian streak decries.
Now, of course, that's my personal reading of Hizdahr on the show. It could be that the actual intentions of the writers were just to make him a buffoon against which Dany's strong-female-character-ness could be highlighted. But what kind of strength, precisely, would be highlighted by a buffoon? "Standing up to" a character who's a joke is, itself, a joke. So I have to believe that the writing intentionally made Hizdahr more sympathetic to show that Dany was out of her depth in Slavers' Bay.