Is it common to get Acute Myocarditis after a common cold? Specifically, more than likely a Rhinovirus. If so, is mortality high?

No. Myocarditis is a rare disease, but not extremely rare. It does happen to people, but it is hard to find any exact numbers about it. Maybe one's chances of getting it are 1 in 5000 or 1 in 10000 people... these are just rough estimates. Likewise, it's not known how many cases are happening after Rhinovirus infections.