I have left over mascarpone that I don't want to waste
Although my own instinct would be to try using the mascarpone, there are undoubtedly some recipes that might not work.
Consider whether the moisture content, texture, or flavor of ricotta is essential for what your recipe is trying to achieve. For many dishes, such as lasagna or other baked pasta, mascarpone should be no less delicious than ricotta.
However, if your recipe truly needs the specific texture and more subtle flavor of ricotta, then it might be worth waiting until you have some, and make something else with the mascarpone instead of wasting it.
Part of the fun of cooking is experimenting with the ingredients you have on hand, and imagining in your mind how they might taste together. If you can imagine the taste of mascarpone in the dish you're making, then absolutely, you should go for it — it might turn out to be your own special creation!
If you use the mascarpone, I would love to hear what you made and how it turned out!