There are several criticisms against the Bitcoin dominance index. It measures market cap in a way that tokens like GNO are double counted. Although the price is multiplied by circulating supply of currency to get market cap, the circulating supply of altcoins are often lesser than shown because a lot of the altcoins are held back by the developers.

          In short, although Bitcoin may be coming down on market cap %, it is more liquid than altcoins and is less likely to inflate due to increase in circulating supply.