1. The ND filter doesn't change the final appearance of the image.
It doesn't have any effect on colours, contrast etc.
It's just cutting the light which forces a slower shutter speed than otherwise achievable.
This effect is only of limited use for drone photography.
2. This is not applicable to DJI consumer drones at all as their lenses are incapable of shallow depth of field at any aperture (and all except the P4 pro and M2 pro are always at full aperture anyway.
M2 Zoom when 2X zoomed in to 48mm equiv does have some shallowness to the back ground of it depending on focus point. Though it is technically a variable aperture lens (only in the sense that it's f/2.8 at wide and f/3.5 at tele, don't think you can set it to f/3.5 at wide).
ND filter can change the final appearance, but usually only if they're poorly made (hence not doing a good job at being 'neutral').
Crusader's comment makes more sense when considering all photo equipment but as you said, most of the mavic line (hence what this forum is for) has fixed apertures with the exception of one flagship mavic model, and the phantoms. When it comes to videography in general (not specific to drones), ND filters, at least a 3-stop and 6-stop pair, are a necessary component to have in a bag.
My take on ND filters as it pertains to fixed-aperture DJI drones under the
Mavic 2 line...
It's primarily useful for video. The 180-degree shutter angle rule to help frame-to-frame transition seem more natural to the human eye. It's not very natural when every frame is tack sharp and there's enough movement that there's nothing to bridge between them (hence why higher frame rate footage, without prolonging shutter exposure appears smoother, there's more frames between the gaps of movement). The effect would be the same in dimmer environmental conditions if we could achieve 1/(2x frame rate) shutter speed if we didn't have to force the light coming in darker.
For photos, it's not quite as useful, a higher shutter speed would be desired anyways given the fixed aperture and movement of the drone. While the gimbal is pretty impressive, it helps to have a faster capture. And the shutter speed on most of the drones can't exceed 2 to 4 seconds long anyways, and that's only ideal if the drone can keep perfect position even with a little breeze (ie: it's enough that you could get traffic light streaks pretty good, if the sensor quality is good enough not to have extreme level of noise at higher sensitivity for say night exposure). Likewise circular polarizers are hit-mostly-miss on a drone since you can't rotate or adjust it once it's up in the air like you can with a camera in your hands.
My goal, more or less specific to drones (mine being a
mini 2, but could be applicable to an
Air 2 even), is that I'm going to mainly use it for videography. The camera and sensor quality is no where near my non-flying cameras, but the drones have the advantage of getting different vantage points, even if their results are inferior. Being a fixed aperture camera, with the intent of being used for videos, the ND filters are a must, especially if wishing to avoid the appearance of jumps, jitters, jagginess due to the lack of motion blur bridging the frames.