We will know more after the RNC convention when those numbers can be compared to both 2016 and the 2020 DNC convention numbers.
There are many reasons the DNC convention ratings could have been down from 2016 from the innocuous more people watched online, the low-energy lack of convention goers had people not as eager to watch, or the prerecorded speeches turned people off, to the hair on fire Biden sucks and no one is gonna vote for the Biden/Harris ticket reason.