The opposite.

From the evolutionary point of view, a virus has no interest in being deadly: if it kills its host, it can't reproduce itself.

When social distancing (quarantine and lockdown) is enacted, it is harder for a deadly strain of the virus to reproduce and hence survive, as we react to it by putting into place stricter countermeasures. Which strains will thrive? The ones that able to "sneak around" unnoticed, by causing only mild symtomps.