Yes and no. Siting a run on fresh growing grass is ideal in the summer months as it doubles as a comfortable substrate and food. Move the run daily so that it doesn't become muddy or soiled. However, cut grass starts to ferment and can cause gastric disturbances and bloat. This means it's not ideal for bedding, neither is it particularly warm and insulatory when the weather gets cold. Try using shredded paper from your recycling on the floor of the hutch. Also, consider providing a large cardboard box filled with straw for the guinea pig to sleep in, rather than fill the entire hutch with straw.