The "flaws" of the characters around him are there less to make them unappealing to the audience than they are to justify why they need this pathetic creep
I'm not sure how you can separate the flaws from the characters themselves. Take away Megumin's obsession with explosions and what do you have? A young mage that is culturally chuuni. Considering her people are all pretty much culturally chuuni mages, you lose a lot of her motivation, some of her conflict hooks, and turned her into yet another loli tsundere. I'm not saying she doesn't have tsundere aspects now, but if you take away the obsession, the tsundere traits get greater focus.
As for why they need Kazuma,
I don't think the girls in Konosuba really need the protagonist. They need somebody to balance out their personality flaws and keep them on track, but just about anybody would do a better job than the guy they've got.
Yes and no. Aqua cannot be trusted with money, or the ability to go into debt. Darkness cannot positively fight against opponents. Megumin can potentially end one battle per day, with massive collateral damage.
So you need someone who can control the finances with an iron fist, deal all the damage to enemies that are not taken care of in one explosion, and lead the mission critical/ majority of enemies into the path of an explosion. Why would such a person join a party of crippled specialists when they can join a more functional party? Or maybe just solo until they find a good party, since they have the combat ability for that. Only Kazuma is desperate enough to stick with them, lazy enough not to try and find something better, and lucky/smart enough to make the party work, while picking up skills to fill the gaps. That said, he doesn't really balance out the flaws of the girls exceptionally well, and has a decent amount of flaws himself.