Griezmann made the move to Barca in the summer of 2019, but he has since struggled to show the form he produced at Atletico on a consistent basis.
He has had decent spells in the current campaign, but Barca as a whole are struggling to make their mark and Mathieu feels it was a mistake for his fellow Frenchman to swap the Wanda Metropolitano for Camp Nou.
“Griezmann is not at his Atletico level,” Mathieu told RAC1. "Personally, in my opinion, it is a mistake that he came to Barcelona.
“At Atletico he was a very important piece and I don't know if he has that importance at Barcelona.”
Mathieu backing for Lenglet
Clement Lenglet came in for fierce criticism after conceding a late penalty against Cadiz on Sunday to cost Barca two points, and the French defender reportedly left the ground in tears.
Mathieu had a tough time in his three seasons at Barca, and he can understand what his fellow Frenchman is experiencing.
“I had a very bad time in my last year with Barcelona," Mathieu said. "I felt that the defeat in Turin [in the Champions League against Juventus] was my fault.
"But I don't understand why they went for me as there are 11 players in a football team.
“I felt alone in the dressing room. No one offered support. For me, that's not football.
"Everyone is killing Lenglet now, and I know what that's like. He has to try and be positive and to turn it around, and try his best."
A big fortnight ahead for Barca
Barca trail leaders Atletico Madrid by eight points and have away games at Elche and Osasuna either side of a meeting with Sevilla in La Liga.
The Catanans also face Sevilla in the Copa Del Rey, with all those games preceding a meeting with Paris Saint-Germain, where they must overturn a 4-1 deficit from the first leg if they are to remain in the Champions League.