A crop lowers total resolution
If a 4000×3000 photo is cropped to 1600×1200, the remaining area still contains real source detail. However, it has fewer pixels available for a large print or high-resolution display.
Enlargement reveals the limit
Problems appear when a small crop is scaled beyond its natural dimensions. Interpolation can smooth edges, but it cannot recreate texture or focus that was discarded. Start with the highest-resolution original available.
Export settings also matter
A crop can look worse because it was saved with low lossy quality, not because of the crop itself. Use a suitable output format, inspect the exported file, and avoid repeatedly reopening and resaving a JPEG.