Yes, no correlation between video quality and SD card. The video might only has a lot of discontinuations as the card is not able to store all data in such a short time.
I don't know which Phantom do you use but for the Phantom 4P I can recommend the fastest card that you can get.
The San-disc extreme pro is absolutely fast enough. I had problems with San_disc ultra in my MavicAir.
But again - no relation with the picture quality.
Did your drone camera have any impacts in the past? Just put one image up here and we can see what's the problem..