I don’t know if this helps but i’ve noticed same thing. But for me it’s really depending on the locations where i fly. I think the big cause is that the bing maps texture quality varies much depending on where we fly.
I could be wrong about your case here. I hope Asobo find the cause of your issue.
I also know it was some time ago there were an issue with server of textures and we got blurry textures in both worldmap and in flight. Check on worldmap if you get bad textures there too, then it could be server issue.
Source of old fixed issue (this i think all of the users experienced, Not sure):
Try also turn on photogrammetry if you have that turned off.