Yes, you can use maple syrup or other liquid sweeteners like agave nectar in this recipe. However, keep in mind that replacing a granulated sweetener with a liquid one may affect the texture and moisture content of the muffins. If you decide to use a liquid sweetener, you might need to reduce other liquid ingredients in the recipe to maintain the correct balance. Alternatively, you could slightly increase the amount of dry ingredients. It might take a little experimentation to get the proportions just right, as different liquid sweeteners also vary in sweetness level compared to granulated sugar. Start with replacing the sugar with an equal amount of liquid sweetener, then adjust as necessary for taste and texture.