The association is established; acting on it is not advised
MTHFR is the most overstated gene on the internet. The variant is real, its effect on an enzyme is real, and almost everything sold on the back of it is not.
The MTHFR C677T variant reduces the activity of an enzyme in folate metabolism. That much is not in dispute, and people with two copies do have measurably higher homocysteine on average.
What does not follow is the shelf of products. No guideline recommends testing MTHFR before choosing a folate supplement, no guideline recommends a special form of folate on the basis of it, and the trials that lowered homocysteine with B vitamins did not reduce cardiovascular events. The gene is used to sell a solution to a number rather than to a problem.
The advice that does rest on evidence has nothing to do with genotype: anyone who could become pregnant is advised to take folic acid, at the same dose, whatever their MTHFR result.
Nothing on this page recommends taking anything. If a result here matters to you, take it to a doctor or a pharmacist rather than to a shop.