Mortality rate for COVID-19 is about 2-3% with medical care, and maybe 10% without. For common flu it's roughly 0.1% depends on when and where of the outbreak.
Most mild cases are reported as they perform temperature check for everyone who enters and exit most if not all enclosures, so unless those people self-quarantined and never go out, they are likely be included in the statistics.
Asymptomatic cases are extremely rare, if they infects multiple others, they will likely be identified through retrospection. Overall, I'd say those cases are unlikely to result in significant difference in the estimate.