This is bad in the short term for Bitcoin. Because Dell uses Coinbase(instead of accepting and keeping Bitcoin directly), coinbase will need to eventually sell the BTC to cover the USD they gave to Dell. This is why the price is dropping. Then people start to panic and sell as well.
I'm holding long term so this is a nice news, it's true that in the short term the selling pressure will rise a little but the adoption will increase at the same time.