I would strongly recommend against doing anything dramatic, such as resigning or publicly denouncing the course. I feel bad about discouraging acting on your beliefs, but I think it could actually hurt your career. There are two reasons for this:
First, the topic of how to teach low-level mathematics courses has become contentious and politicized in recent decades. Unless this is actually your scholarly specialty, it's safest not to get too dramatically involved, particularly as a grad student. The problem is that you can easily find people who will embrace you as a champion or martyr, whichever side you are on. Even if you are sober and self-restrained (which would probably rule out a public letter), other people on your side will say provocative things and attract negative attention while supporting you. Controversy is dangerous for academic careers, since it's generally easier to veto hiring someone than to generate an offer. So if you acquire equal numbers of friends and enemies, your enemies can hurt you more than your friends can help you. Plus, if you want a research career, being known for inflexible opinions about low-level teaching will distract attention away from your research accomplishments. That distraction can be a problem even for people who agree with you.
Second, you risk coming across like a worrisomely disruptive colleague. Most math departments contain at least one faculty member who regularly takes fervent stands on seemingly minor issues. They feel they have logically analyzed these issues, and they can't in good conscience cooperate with anything other than what they see as the logical option, since that would be a betrayal of the basic principles underlying mathematics. Coordinating with others or compromising play no role in the analysis, and it doesn't really matter how important the issue itself is (what matters is standing up for what's right). These people drive everyone else nuts, since they make it impossible to get anything done without either giving in to them about their pet issues or spending hours debating.
I'm not saying you are necessarily disruptive in this way. You have chosen an important topic to get upset over, and you might be completely right about it. However, if a hiring committee hears that you resigned in disgust upon being asked to administer online quizzes, they will wonder what else you might make a fuss over. This could put them off even if they agree with your concerns about teaching, and there's no way to reassure them that it's really just this one issue.
So what can you do while avoiding these dangers? One approach is to let the faculty handle this fight. If every faculty member disagrees with you, then your cause is hopeless in the short term and it's best just to calm down and finish your Ph.D. program without too much controversy. If some of them do agree with you, then it's not likely that publicly joining them as a grad student will shift the balance of power in the department. Instead, you can try to get your future TA assignments in courses they teach, while encouraging them behind the scenes in their attempts to change the department's approach.
To the extent you take direct action, I'd look for approaches that don't cause extra work for anyone else. For example, if you resign, then someone will have to find a replacement for you (so they'll automatically be upset about it). But you might be able to improve the course by strategic volunteering. Could you prepare optional handouts meant to deepen the students' knowledge? Could you offer a few additional review sessions before exams? These sorts of things aren't going to effect the fundamental changes you seek, but they could at least make you feel better about having done something rather than nothing, and they may build some goodwill with the lecturer by showing that you really want to help the students.