According to Kaiser Family Foundation:
The 2010 Affordable Care Act (ACA) modified current law related to the Part B income-related premium.10 The law imposed a freeze on the income thresholds that were in place in 2010 from 2011 through 2019, rather than allowing the thresholds to rise with inflation.11 This provision will increase the number and share of beneficiaries who will pay the higher income-related Part B premium over these years. In 2020 and subsequent years, the income thresholds will once again be indexed to inflation as if they had not been frozen between 2011 and 2019.
So the answer today is no. They are not indexed to inflation. However, due to federal budget reconciliation rules, they will be indexed to inflation in 2020 and afterwards as if they were never not indexed to inflation. I.e. 2020 will be at the inflation-adjusted rate of 2010. Presumably that will be a big increase from where it is now.
Of course, Congress could change the law again between now and 2020. This is the current law, not a binding contract.