Why is Lightroom Classic reducing the quality of my photos?

The image you see in the Finder (?) is not the raw file, but an embedded jpg preview created in the camera.

Any settings you have enabled in the camera, like sharpening, noise reduction, Picture styles or similar, are applied to the jpg. These settings are also included in the raw file, but Lightroom doesn't understand these proprietary settings, and ignores them.

So you have to edit the image in Lightroom to improve it.

Raw files are open to interpretation, and Lightroom only gives you a starting point, the rest is up to you.

Lightroom is a professional image editor, so it is entirely possible to make the image as good as, or even better than the jpg preview.

When evaluating image quality, always view the image at 100% view, where one image pixel is represented by one screen pixel.

Any other view will be inaccurate and misleading because the image has been scaled.

Zooming in beyond 100% will inevitably result in pixelation.

To make proper screenshots, see https://www.take-a-screenshot.org/mac.html

It's difficult to evaluate based on that image. The image appears to be not in focus, not properly exposed, and from what you have said, you haven't attempted to do anything to it using Lightroom. So it's difficult to make a suggestion considering just this particular image. Sorry.

Lightroom cannot "reduce" the quality of an image. It can only display what is there. What are the pixel dimensions of the image? In the screenshot that you posted, are you displaying the image at 100% magnification, normal view, or what? Have you worked on the image to optimize it in Lightroom at all, or is this just what it looks like after it was imported?

Is this a JPEG or a raw image? It takes a little work to optimize images. I took some images at a wedding of my daughter last week with the ISO was set at 4000. It took a little work to make the images look the way I wanted them to look. It's not a good idea to blame everything on Lightroom, I can attest to that.