I don't think it's a good idea. When the price of Bitcoin drops, most alt prices will also go down and indeed the price of Bitcoin greatly affects the market. If you really want to invest or trade alt then from the start we can buy Alt in addition to buying Bitcoin. So we can buy both without having to sell one of them. Choose an altcoin that has good prospects so that it can make a profit. Bitcoin and altcoin investments will be equally profitable depending on how we manage the assets that we have.
Well, its always expected since bitcoin still dominates the crypto space and whatever its price will always leave a huge impact on altcoins. If you can have all the means to buy bitcoin and altcoins at the same time, then why not? They are both profitable so they deserved to be kept and hold.
However, if you don't have enough money to invest in altcoins, then maybe selling a part of bitcoin is not a bad idea at all. The fact that you are doing this because you want to accumulate more profits in the future since more investments will mean more profits, so definitely there is nothing wrong with it.