Firstly, the Fed is purchasing agency MBSs at market rates. They are pushing prices up since they are bringing demand to the market, but they are otherwise bidding competitively. The idea that these MBSs are basically worthless doesn't make sense when there is a large market for them.
Secondly a lot of the MBSs (which are all government guaranteed) are newly issued, so it can hardly be said they are helping TBTF banks decant their toxic assets leftover from the financial crisis.
Thirdly, the Fed actually eventually made a profit on the toxic debt they acquired during the bailouts. The current MBS purchase programs are far less risky. Furthermore, the US Gov has guaranteed the future cashflows of all these MBSs... so I think TBTF banks are a sidenote on this whole issue.