Procedure for Small Mouth Opening

A true congenital condition of a small mouth is called microstomia. This is relatively rare, The most common cause of acquired microstomia is either due to cancer removal or electrical injury. In either of the two later cases an anguloplasty or oral commissuroplasty can open the outer corners of the mouth but will tend to leave visible scars.

If you have no congenital abnormality or surgical or traumatic injury, then there is nothing you can do and look normal.
Lip lifts can cause unsightly scars and may not give you what you want.

Although patients who have had traumatic injuries and birth defects with a small, tight mouth generally are happy with the results of surgical procedures, it is not something that someone would be happy with who has just a cosmetically small mouth.