Not sure if this is still a problem, but if I understand your issue correctly you can customise the “Download Plot Button” via the plotting configuration. More specifically by increasing the ‘scale’ field under ‘toImageButtonOptions’ it downloads png with greater resolution (tested in the browser in a jupyter notebook - kernel python3.9, on macOS Big Sur). For me setting the ‘scale’ field to 20 did the trick.
For details on how to change the plotting config in python (must be similar directly in js) see Configuration | Python | Plotly