No, thick yoga mats are not bad. In fact, they can be quite beneficial for your practice.
Thick yoga mats provide extra cushioning, which can be helpful if you are practicing on a hard surface. They can also help to protect your joints from impact.
If you are new to yoga, a thicker mat may help you to feel more stable in your poses. It can also be helpful if you are practicing yoga on a slippery surface.
However, thick yoga mats can be more difficult to transport and may not be as comfortable to carry around. They may also be more expensive than thinner mats.
ultimately, the decision of whether or not to use a thick yoga mat is a personal preference. If you are looking for extra cushioning and support, a thick mat may be a good option for you.