Fit of Fill Zoom View settings cause LR to apply a 2nd interpolation to the preview, which can soften the image. Try viewing the Export images at the Pyramid Zoom View settings (1:8, 1:4, 1:2, 1:1). If still experiencing loss of quality using these Zoom settings please provide the following information:
OS version
Camera Model
ISO images were shot
Develop module Sharpening and Noise Reduction settings,
Export module File Settings, Image Sizing, and Output Sharpening settings
A screenshot showing the export image and raw image at same size onscreen using Compare mode would also be helpful.