please have a look at these comments on interferometric quality of Sentinel-1 products:
- Coregistration with Back Geocoding does not work
- Phase to elevation vs phase to height
- Some explanations about concepts of fringes of interferogram and coherence
- Extensive areas in interferogram with unexpectedly high interferometric phase - the result of atmospheric artefacts?
Good materials on error sources of InSAR products: https://eo-college.org/resources/insar_errors/
What is the temporal and perpendicular baseline of your image pair? The only thing you can do to improve the interferogram quality is to select suitable images: Help in DEM Generation using Sentinel-1
There are some excellent documents on the selection of suitable images for interferometry by ESA: Source Section B- 1.3 Selecting images for InSAR DEM generation.