How is the quality in the sonar viewer?
A couple of things to bear in mind with mosaic quality. The first is that mosaics are shown at a lower resolution in the map view due to the very large amount of data that can be involved, especially when combining multiple tracks into a single mosaic. When mosaics are exported, they are rendered at full quality. Another consideration is how much the sidescan image has been stretched when slant-range correction is applied. If you want to post a screen shot of your images I may be able to advise further.