Could you please share a bit more about your scenario?
Do you want to use MM Placeholder to represent the Month instead of mm Placeholder?
Currently, within PowerApps, we could use mm Placeholder to represent the Month or Minute when formatting Date time values.
Actually, we should not use mm Placeholder individually when formatting Date time values within PowerApps, it would make no sense. You should use mm placeholder along with Year placeholder, Day placeholder or Hour placeholder.
In other words, the value the mm placeholder displayed as would be based on date time format you used:
Text(Now(), "mm/dd/yyyy") /* <-- mm placeholder would display Month value */
Text(Now(), "hh:mm") /* <-- mm placeholder would display Minute value */
More details about the Date and time placeholder, please check the following article: