Yes, it is generally possible for someone with a height of 5'4" to ride a KTM RC 125 comfortably. The KTM RC 125 is a sportbike with a relatively low seat height, which makes it more accessible for riders with varying heights. However, personal comfort and riding experience can vary from person to person, so it's always recommended to physically sit on the bike and test it out before making a purchase.
When sitting on the bike, make sure you can comfortably reach the ground with both feet flat and have a relaxed posture while reaching the handlebars and controls. This will ensure that you have proper control and maneuverability while riding. If possible, take a test ride or consult with a dealer to get a better understanding of how well the bike fits your body size and riding style.