I'll speak only for these two because they seem the most isolated from the fandom, and they're not wrong. Like damn Naoya and Mahito did horrible things to earn people's hatred... but they deserve recognition for fulfilling their roles as villainous/infamous characters well.

Don't get me wrong, I'm not minimizing or trying to justify what they did, but their value as characters can't be measured simply because: "he does bad things = shitty character, doesn't deserve to exist, how does anyone like him?"

Think for a moment, what made Mahito's defeat by Yuji in Shibuya so special and satisfying? And it's not just because he deserved it; with his defeat, Yuji completes a stage of development that has been unfolding since the beginning: **understanding and accepting his responsibility as a sorcerer**. With Junpei's death, he realizes that there will be people or situations where words won't be enough. Nobara's "death" and Todo's speech in that horrible situation on the subway would make him understand Nanami's last words and find the courage to follow them.

The same goes for Naoya. Through Maki we learned about the mistreatment within the Zenin clan, and with Toji we saw the consequences of it. But with Naoya we saw both: a child raised in a terrible environment where treatment is determined by how you're born ended up becoming a repugnant person with twisted values.

That's why his battle against Maki is the perfect end of the clan; Naoya embodies it and everything Maki hates about it. When he returns as a curse, serves narratively as an aid for Maki to complete her Heavenly Restriction, and I even find his life and death poetic. Naoya was born to be one of the best sorcerers, but he admired someone he couldn't even use jujutsu. Despite being the best he was defeated by the clan's failure and murdered just as he said women should live. He was reborn as a curse, proclaiming with even greater force that only he could be considered Toji's equal, and he died giving rise to the woman truly comparable to Toji.

You are free to have your own opinion about them, but denying their importance and value in the series just because they acted according to their role in the story is a very poor and childish judgment. I know that Mahito's greatest adversary was a child he could only face if he mentally destroyed him, and Naoya only had about 3 fights and lost them all, lol, but it is their characterization and participation that make them memorable and enjoyable. 

They are some of my favorite characters, not because I approve of what they did, but because they were useful characters with a purpose, and their actions were where the story I love so much was built.

Some appreciation for the villains, they literally do their job but they do it so well that they only receive hate 😆