Using light weights can be effective for maintaining muscle mass, especially if you are currently at a fitness level where lifting heavier weights may not be appropriate or if you have an injury or medical condition that requires you to use lighter weights. While lifting heavier weights is generally considered more effective for building muscle mass, using lighter weights can still provide a good workout and help you maintain your muscle mass.
To make the most of using lighter weights, you can perform more repetitions (i.e., higher volume) to challenge your muscles and create the stimulus needed for them to adapt and maintain their size and strength. Additionally, incorporating exercises that target multiple muscle groups (e.g., compound exercises such as squats, lunges, push-ups, and pull-ups) can be more efficient in terms of maximizing muscle activation and time spent in the gym.
It's also important to note that maintaining muscle mass isn't just about lifting weights. A well-rounded exercise program that includes cardio, flexibility, and balance training, as well as a healthy diet, can also help you maintain your muscle mass and overall fitness.