When you import your image sequence into Lens Studio, there are two important settings that affect image quality: Files count and Single texture maximum size. Lens Studio will generate animation sprite sheets for your sequence based on these settings. If you leave it as default, it will compress all your PNGs in that sequence into one sprite sheet that is 2048 by 2048. This might be what you are seeing.

To get better quality, increase the Files Count to a higher number so that your sequence is spread out across several sprite sheets. When increasing the Files Count, do take care to not exceed the single Lens size limit of 8MB. A rough estimate is that one Lens can have two to three 2048 by 2048 textures or five to six 1024 by 1024 textures. Check out the 2D Optimization guide for more information about 2D animation optimization.