The KISS singer tells TMZ ... he likes JoJo's new "demon" look, praising her edgier new vibe as badass, and adding ... anyone hating on her, simply doesn't get it and they might just be jealous.

Gene also says, "Never be ordinary. Always be extraordinary. JoJo is extraordinary and she looks cool."

Gene's cosign comes after JoJo hit the red carpet Monday in an all-black mesh bodysuit at the iHeartRadio Music Awards.

She rounded out her eye-catching ensemble with dramatic face paint -- similar to what KISS bandmembers wear -- and a theatrical faux hawk.

JoJo announced she was entering a more adult era of her career earlier in March, as part of the release of her new song, "Karma." While many have struggled to embrace the "Dance Moms" alum's darker style, JoJo seems committed to the over-the-top getup moving forward.

Luckily, she can do so knowing Gene approves -- and even though he admits she looks like him in makeup, he's not really bothered by it one bit.

Remember, Gene and his Kiss bandmates recently semi-retired their iconic onstage attire as they wrapped their farewell tour this past December, even though their performances (and their costumes) will live on digitally -- an innovation they announced as a band not long ago.