There would be no inherent problem with having the Fed hold this much debt, if the tax system then soaked up distortions in how the resulting stimulus was distributed into the economy. That is, if the tax system is structured to pull back currency from unproductive sinks before it yields inflation, then there's no problem with the Fed holding that debt.

A better structure, as others have pointed out, would be to have the Fed just hold the government's debt, so that allocation and taxation can both be managed by the government. At the moment, the Fed and Treasury (via Congress) are both competing to play an allocating role in the economy at the same time, with no real coordination or plan.