Preview and exported images look different because the demosaicking and interpolation algorithms used are different. Even when viewing the exported image at reduced magnification, the down-sampling algorithms are likely different (nearest neighbor vs bicubic vs cubic vs lanczos).

Consider adding a final sharpening step after any additional post processing you plan to do. If you intend no further processing, you can add additional sharpening from within darktable.

Preview and exported images look different because the demosaicking and interpolation algorithms used are different. Even when viewing the exported image at reduced magnification, the down-sampling algorithms are likely different (nearest neighbor vs bicubic vs cubic vs lanczos). Bicubic and cubic will tend to look softer.

Consider adding a final sharpening step after any additional post processing you plan to do. If you intend no further processing, you can add additional sharpening from within darktable.