Arnold called Barr "a racist" and accused her of being a conspiracy theorist in an interview with Good Morning Britain on Monday.
He said that although he hasn't spoke to Barr in years, her past six months of social media activity has proven her to be different from when they were married, from 1990 to 1994.
"I see her social media and she's one of those conspiracy people, there's a lot of them in America," he said, before likening her to President Donald Trump. "Our president, you know, he's a racist, he feeds into this conspiracy stuff, there's so much anxiety in our country."
"I see her social media and I'm like 'Oh my God, she thinks Donald Trump is about to arrest Hillary Clinton, Barack Obama and send them to Guantanamo,'" he added.
Arnold, who worked as an actor and writer for the original Roseanne show in the late '80s and early '90s, also said Barr should "get off Twitter."
He said that he saw the scandal coming and that ABC should have gotten involved in Barr's social media antics long before last week.
Previously, Arnold told The Hollywood Reporter that he thinks Barr's outlandish social media presence was a calculated effort on her part because she "wanted" the show to get canceled.
"It had to happen," Arnold says of the show's abrupt end. "And I am going to tell you the truth, she wanted it to happen, if you saw how her tweets escalated this weekend. If it hadn't happened yesterday, this season would have been so awful for everyone every day because she would have felt like she was [being] taken advantage [of], just like when I left the show."
He added that the simple solution to the problem would have been to take away Barr's phone.
"ABC lost maybe $1 billion from this; this show was grinding out money hand over fist and they lost it all because somebody didn't say, 'Get that phone out of her hand,'" Arnold told THR. "She's not going to go on TV and say these things. But you put that phone in her hand and she is a loose cannon."
ABC pulled the plug on Roseanne last Tuesday following a tweet from the 65-year-old creator, head writer and star of the show that compared former Barack Obama adviser Valerie Jarrett to an ape.
Public outcry was swift and harsh following her tweet. Barr has both apologized profusely and staunchly defended herself, even at one point explicitly writing on Twitter that the initial tweet was "not racist." She also blamed Ambien, a prescription sleeping pill, on the tweet.
Barr reportedly did an interview for a friend's podcast, but podcast host Rabbi Shmuley announced that he will not release the podcast until she decides she wants it released.
"I did record a podcast with my friend Roseanne, [and] I have decided not to release it out of respect for Roseanne. I want to give her space to reflect on the recent events and releasing the recording is a decision she will make at the appropriate time," he said.