Of course it's going to badly impact the US dollar, just how severe should be the question you should be asking.
It won't as the poorest citicens won't use that BRICS,
40% but 80% of the poorest citicens live in the BRICS countries,
That's a weird percentage you came up with lol
In any case BRICS and its currency has nothing to do with "citizens" and their day to day medium of exchange. None of them are using dollar today and they won't start using BRICS currency in the future either.
BRICS currency is something meant for international trades on very large scales (trillions of dollars) between countries that is currently taking place using dollar. That is why its creation will have a negative impact on US dollar as it will lose its demand and with dollar's massive circulating supply and US unimaginable national debt the dollar value could crumble.