Myocarditis is a rare side effect of both COVID infection and the COVID vaccines. It is a rare condition in general with an incidence around 10-20 cases per 100,000 people (for reference a disease is typically considered rare if it affects less than 50 per 100k). You will often see myocarditis risks listed as x times more likely in this group vs this other group, but what is often left out is how rare those cases really are.

"the risk for myocarditis was 0.146% among patients diagnosed with COVID-19 during an inpatient or hospital-based outpatient encounter and 0.009% among patients who were not diagnosed with COVID-19" (CDC).

There have been a lot of studies looking at the risk of myocarditis and vaccines and it is true that they increase the risk in many cases many times more than the general population but again keep in mind that it is a rare condition to begin with and often such studies require massive sample sizes to only have a dozen or so cases of myocarditis.

Here is a recent study (n=404,407) which found 15 cases of myocarditis that could likely be linked to vaccination.

"The risk estimates of myocarditis among male recipients in the 21 days after the first and second doses were 0.56 cases per 100,000 after the first dose and 8.09 cases per 100,000 after the second dose; the risk estimates among female recipients were 0 cases per 100,000 after the first dose and 0.69 cases per 100,000 after the second dose."

It's hard to compare these directly but if you compare these findings it would seem the risk for myocarditis is around the same (9 per 100k) for both mild infection (which is overwhelmingly the case for adolescents) and for vaccination. In both cases it is extremely rare and generally mild. But in the vaccination group there is no risk of long COVID, the small but very real risk of death or ICU admission and permanent organ damage etc.