To me, the eyes seem kind of like a stereotypical hypnosis swirl, which I think is sort of on-theme for Jynx, although her Pokédex entries always attribute her hypnotic abilities to the way she dances. Maybe the idea was that by Mega Evolving, her hypnotic powers would increase and be transmitted through her vision as well?
I wish we got a higher quality image of Mega Jynx, since it would help determining what the eyes are supposed to be. there could be an argument that the black/grey aspects are supposed to resemble sunglasses, there also seems to be some extension of the white aspect on the purple face, so the white part could be the actual eye. This would probably mean Mega Jynx is leaning more into Fashionable Snow Queen which would pair nicely with the fur coat/boa like hair growth.
However if the black aspect is Mega Jynx's actual eyes, then it could probably supposed to be evil or sinister eyes. Which would make Mega Jynx lean more into the Evil Ice Queen trope. Which would fit both inspirations of how the Snow Queen and Oni Woman (Yama Uba) are popularly depicted.
Honestly though I wish they kept the Yellow/Purple/Red color scheme Base Jynx had, since that color scheme is quite unique to Jynx and makes it stand out among the other Ice types. If they made the dress Red and used the Light Blue color for the edges of the dress to resemble frosted edges or used as gradient on the hair to distinguish the fur coat/boa-like hair from the rest, it would probably make the mega fit more with Jynx and make it feel less jarring.