The dominance and marketcap are more accurately reflected real things in bear market. In such market phase, altcoins will be pulled back to their real values and scam projects will be killed to 0. Their contributions to total marketcap will be dropped significantly and scam project will have almost zero marketcap contribution.
I guess you meant low volume coins, well such projects will be called scam projects if killed during bear market by their marketcap turning to nothing but 0 but the recent bitcoin low dominance is as a result of altcoins such as Doge that increased to over $34 billion in marketcap, also is ripple that increase very significantly that its marketcap almost increase four fold recently including some good altcoins. The bull-bear-marketcap relationship can be somehow at times because bitcoin dominance during bull run at one time this year was over 70% but the altcoin season lower it to around 50%, but you are correct about saying that the low dominance will later fall more on altcoins if bear market begins.