Kashrus organizations are for the purpose of determining that the ingredients don’t contain foods and/or byproducts of the foods (fish/meats etc) enumerated in Torah shebeK’sav and Baal Peh to be impure/unfit (in the common vernacular “treif”) as well as basar b’cholov.
If you want to know what’s healthy see a doctor.
They wouldn’t allow a mixture of fish and meat to be sold.
That’s a sakana issue.
However, something which doesn’t reach the level of sakana, but is “unhealthy” is okay to eat in moderation for most people, so it’s okay to give a hechsher. How much to eat and who should avoid these foods entirely is indeed a discussion one should have with their doctor, but is not the hechsher’s responsibility.