The Yamaha FZ V3 is indeed considered a muscular bike, but its suitability for you depends on various factors, including your personal preferences and comfort. As a slim person with a height of 5'6" and a weight of 50kg, you may find the FZ V3 to be slightly on the larger side, as it has a seat height of around 790mm and a relatively bulky appearance.
While the bike's dimensions may not perfectly match your physique, it doesn't necessarily mean that it won't suit you. It's essential to consider your comfort level while sitting on the bike, reaching the handlebars, and placing your feet firmly on the ground when stationary. You should visit a Yamaha dealership or a showroom to sit on the bike and get a feel for it yourself. This will give you a better idea of whether the FZ V3's ergonomics align with your body proportions and if it provides a comfortable riding position for you.
Additionally, keep in mind that riding comfort can also be influenced by factors such as the duration of your rides, the type of roads you usually travel on, and your personal riding style. It's always a good idea to test ride the bike or seek the opinion of experienced riders who have similar body proportions to get more insights.
Ultimately, the decision of whether the Yamaha FZ V3 is suitable for you or not depends on your personal comfort and preference, so it's recommended to try it out in person before making a final decision.