ND filters again...when will people understand two simple things. First, reducing the frame rate does not make the video smoother. 25 or 30 FPS is not smoother than 60fps unless whatever you play it on can't handle it.
Secondly, that bit of dark grey glass reduces the light reaching the camera, so the ISO, which increases graininess the higher it goes, has to be increased a lot.
100 ISO no grain. 1600 ISO grainy as ****.
..and on the mavic with its fairly awful camera, even ISO200 is fairly unacceptable with noise. Anything over base iso is going to be lower quality.
Im stunned how many people have absolutely no understanding of what an ND filter does. Its a bog standard piece of tinted glass yet people assume they have magical qualities.
Its only "drone world" that people seem to misunderstand these simple creatures.
The OP has stated the video has noise and grain. That is something an ND filter is going to have absolutely no effect on (also wont affect contrast, wont affect sharpness, wont effect highlights and all the other snake oil that gets peddled on here). In fact, if you already have image noise, decreasing light to the sensor with an ND is going to make it worse!
Then you make a situation worse by people using an ND they dont understand with a PL they also dont understand but stick it on anyway with no adjustment.
NDs are used for motion blur yes. But thats only realistically going to be noticed on a mavic if you have a LOT of motion in the scene, flying low and fast for example. For normal, high altitude, gentle shots where there isn't much movement from frame to frame an ND isnt going to be needed.
If you're taking stills they're the last thing you want bolted onto the lens.
Anyway to the OP, are you shooting in 4k? What sharpness settings do you have? On the mavic pro you need sharpness +1 or noise reduction kicks in and smears details. Its a bug thats been around from the first few months.
Contrast and sharpening best left usually at 0 (or even -1) and you can customise it in post processing. That isn't going to change for lighting conditions.
Are you using d-log, d-cinelike or another profile?
We need more detail to offer useful advice. Ideally some sample footage as well.