"She's so cool, and he's not," Adam Levine joked about his pals Gwen Stefani and Blake Shelton, who got engaged last October

Adam Levine has a few thoughts on his pals Blake Shelton and Gwen Stefani's relationship — but Shelton isn't going to want to hear them.

The Maroon 5 frontman, 42, appeared in-person on The Ellen DeGeneres Show Thursday as part of the 3,000th episode, where he cracked some jokes about his former fellow The Voice judges, who got engaged last October.

"I don't support their marriage," Levine teased to Ellen DeGeneres.

"She's so cool," Levine said of Stefani, 51, before adding of Shelton, 44, "and he's not."

"But he's very funny and very smart," DeGeneres noted of the country star.

Levine still only slightly agreed with DeGeneres, 63. "He's vaguely charming," Levine countered, before laughing to himself.

adam, levine, ellen degeneres
Adam Levine on The Ellen DeGeneres Show
| Credit: Michael Rozman/Warner Bros.
adam, levine, ellen degeneres
Credit: Michael Rozman/Warner Bros.

"I missed this. I missed making so much fun of him," Levine admitted, before DeGeneres suggested that the "Girls Like You" singer attend Stefani and Shelton's wedding and do just that.

"I'll go to the wedding and I'll object," said Levine. "I probably won't be invited now that I'm saying this on live television. But I'll be there somehow."

"I'll be your date. Let's go together," DeGeneres suggested.

"Let's go. Let's just crash," Levine added.

Blake Shelton, Adam Levine, Gwen Stefani
From L to R: Blake Shelton, Adam Levine and Gwen Stefani
| Credit: Trae Patton/NBCU Photo Bank/NBCUniversal via Getty

Levine previously addressed Shelton and Stefani's future nuptials during an interview with SiriusXM's The Morning Mash Up back in March, where he joked that the stars "couldn't afford" to have him as the event's singer.

However, Levine said that he "would love" and "be honored" to play at Shelton and Stefani's wedding. "They won't ask me though. They'll probably get like Luke Bryan or somebody," he quipped.

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Stefani and Shelton, meanwhile, have both gone on record to say that the "She Will Be Loved" singer has an open invitation to perform at their ceremony.

"I definitely would love to have Adam sing at our wedding," Stefani said last week on Late Night with Seth Meyers.

RELATED VIDEO: Gwen Stefani Almost Ruined Blake Shelton's Marriage Proposal: 'I Was Trying to Get Out of Going'

Shelton expressed similar sentiments on Meyers' show in December. "I've decided that I've been doing this a long time now and I've got a lot of favors out there. He may not like it, but Adam Levine's gonna have to get the band together and come and play our wedding."

"Their music is so boring that it won't distract from the festivities and the reception," the "God's Country" singer joked of Maroon 5.

Shelton proposed to Stefani at his Tishomingo, Oklahoma, ranch last October. The couple began dating in 2015 after they met as coaches on The Voice.