Anything that contributes to you not needing to actually "think" and instead just "react" is going to be bad for you because it is simply engaging your reward system. The only way LLMs can be a net good is if they free you from drudgery and allow you to work harder on the things that actually matter. (Think dishwashers and laundry machines). If you are using them as an "easy button" so you can finish your work (poorly) to have more time to scroll your timeline then yes, you are turning your brain into mush.

I'm purposefully not engaging with whether LLMs are actually even good at what they do, which is another discussion.

I think this is true of just about any technology. It will make lazy people lazier and help productive people get more done in less time. It's all about where the motivation is for each individual.