Hi,
Show Title is not a feature specific setting it is part of all features that is why it is on the general tab in module settings and you cannot just add arbitrary settings to existing features.
You can add a setting in user.config like this:
<add key="ShowModuleTitlesByDefault" value="false" />
and that will make the ShowTitle setting be false on newly created instances of all features (not just html) but will not affect existing ones that are already true.
You could update all existing Html content instances to not show the title by using a sql query
UPDATE mp_Modules
Set ShowTitle = 0
Where FeatureGuid ='881e4e00-93e4-444c-b7b0-6672fb55de10'
Hope that helps and thanks for the coffee!
Best,
Joe