This will definitely hurt the market as it is a huge amount of bitcoin so it is difficult to avoid negative impacts. But I don't think this is the start of a bear market because Trump is so bullish on the market. So once he officially enters the White House, he will be the one to pull the market back if Biden decides to dump bitcoin on the market.
Even though Biden is still the incumbent president until the end of January, but I think his power is also very limited. Just like increasing aid to Ukraine is also facing many difficulties as Marvell1 mentioned. I don't think he would have any luck and face no resistance if he tried to sell the bitcoins the US government is holding.
I thought that as well, but on second thought i just realized that it doesn't need to. It depends how it's been done, as in most cases big sales like these would be done via OTC that doesn't destroy the liquidity, and those buyers would probably be long time holders, because of the price.
In fact that would give more incentive for those buyers to keep the price up, because they need to protect the value.
Also, Trump would need to start that bitcoin reserve from the start, meaning that it would increase the buy pressure.