Is it a bad idea to use other professors' Youtube videos as lectures?
The answer depends on your goals.
If you have plans to work in academia, eventually you might find yourself on a track to becoming a professor yourself. In this role, you are expected to teach. Hence, you should be able to prepare teaching materials of high quality to support your teaching and benefit students' learning. The sooner you start working on your first video lecture, the sooner you prepare the one which you are actually not totally ashamed of. Your first lecture might be half-baked and otherwise of sub-par quality, but that's expected on your career stage, when people start learning the ropes. But if you listen to feedback from your students and your mentors, and reflect on it, then your second lecture will be better than your first one, and eventually you start producing good quality stuff. Most importantly, you will become a better educator yourself, equipped with a wide range of tools and techniques, and understanding which method works best in a particular situation. You will be able to adapt your teaching to the needs of a particular student cohort, and you won't depend on materials from the internet (which may or may not be available).
But maybe your goal is simply to survive this teaching assignment until you graduate? In this case there is probably no shame in using online materials to complement your teaching. Just don't forget, that you still need to support your students and help them contextualise and understand the content. You may achieve this by preparing some examples based on the materials in the lecture and working through them with your students. It may be appropriate to have a discussion regarding the material, or even to critically assess and compare the material presented by different lecturers.
Good luck.