If the quality of your video is different in the trimmer to the viewer, it could be due to the viewer settings.
Check to see what the playback quality is set to on the Hitfilm Viewer tab, the lower the setting the lower the quality you will see in the viewer.
Lower playback resolution makes it easier for your computer to keep up with playback in real-time. Further details are available in the manual in section 3.8.
You always have complete control over the quality and resolution used to display video playback and paused images in HitFilm, so you can optimize them to whatever balance of performance and quality that you prefer
It is also worth noting that if you are using a Proxy setting, this can also affect the playback quality.
None of these will affect the final output quality of the project you are working on.