Katona has reformed the band with Liz McClarnon and Natasha Hamilton for ITV2's The Big Reunion, along with a UK arena tour featuring the other reunited groups.
The former Westlife singer described the reunion as "funny", calling them "three middle-aged ladies who talk about their families".
He told Now magazine: "It's funny. They look a little different.
"They certainly aren't as, shall we say, fresh as they were back in the day. Then again, none of us are.
"They used to be wild and now they're three middle-aged ladies who talk about their families."
McFadden is father to Katona's eldest children Molly, 11, and Lilly-Sue, 10. They divorced in 2004, before he began a seven year-relationship with Delta Goodrem.
Speaking about whether he still has any kind of relationship with Katona today, he said: "No. Not at all, no. My relationship is with my children and all my dealings go through their nanny.
"They have a nanny who's with them 24 hours a day."
Last year, Kerry Katona told Digital Spy that McFadden had abandoned their children.
She said: "Brian lives in Australia. They see him about twice a year. I think it's quite shocking. As far as I'm aware, they don't even speak every week."
Watch Kerry Katona talk to DS about Brian McFadden below: