Congress did not impeach based on the mueller report.
Congress impeached because of the IG report based on the NSC whistleblower report of the Zelenskyy call, where Trump attempted to blackmail the president of Ukraine to do him a favor in exchange for him approving the sale of a shipment of javelin missiles.
And then again because he led an insurrection against the government he was leading to prevent peaceful transition of power on January 6.