You can change the format of most date/time fields from data sources by using the advanced setting "TIME_DIMENSION_FORMAT_???" -where the ??? is the chosen format, for example: "TIME_DIMENSION_FORMAT_Ym". The string for this is placed under the Options->Advanced settings section of the sidebar, as shown below:
Some common formatting examples:
|Special setting||Basic format||Example output|
|TIME_DIMENSION_FORMAT_F Y||month YYYY||January 2018|
For a full list of formatting options, see: https://www.php.net/manual/en/function.date.php
If you want to use comma between the different date values, you need to write it as in "comma".
If you find that Google Sheets is changing the values in some cases (some users have reported seeing the raw numeric value for the date), try adding this advanced setting as well - "DISABLE_DIMENSION_AUTOFORMAT".
You can add a comma between it and and the format option:
WARNING: Please note that optional setting "Format results for Google Data Studio" overrides the custom date format settings. You must disable that option to get your custom date formatting to come through.