How Important is the GRE General / Subject compared to ACT / SAT?
Certain programs in Mathematics will heavily weight the GRE Subject test for mathematics programs. Many top programs will have a threshold that they expect students to pass for them to actually read the application. Usually, one should view the subject test more as something that can help you be considered, not something that can get you accepted to a program.

This being said, it is definitely something that should not be considered just as a formality when applying to top 25 universities.