Yes—if you are exporting as JPEG, reducing file size by lowering the quality setting also reduces image quality. JPEG compression is normally lossy, which means it saves space by discarding or altering some image detail.

The trade-off is gradual: small reductions from the maximum setting may have little visible effect, but stronger compression will introduce artifacts and reduce fine detail. In practice, many people use a moderate JPEG quality setting for web images because it gives much smaller files with only minor visible loss, while aggressive compression becomes obvious.

This is specific to JPEG. Other export formats can behave differently: some, such as losslessly compressed TIFF, can reduce file size without reducing image quality.

So for Lightroom JPEG exports, the answer is yes: smaller files usually mean lower image quality, and the right setting depends on how much quality loss you can accept for faster uploads.