Perhaps drifting into OMNI PR territory, but it is absolutely the responsibility of the mayor. He got TfL into the position it's in by, amongst other things, "freezing fares" for 5 years to curry favour with voters and win 2 elections. He's been required to cut budgets or raise revenues in some way as a condition of the government providing central emergency funding. He has a lot of discretion as to the way he does so. He has chosen this Travelcard and ULEZ expansion in the hope he can piss off voters and blame it on the Conservative government.