tl;dr: No, it is immoral.
A "non-funded" TA is like a "non-funded employee": A person volunteering to do work which others, typically junior researchers, do for pay, to make a living. TA wages are typically rather meager, with this part of their work often not being extricable from their research, so that another employment option is not academically useful or logistically viable (or allowed); and with graduate employee unions being either missing, fledgling or weak.
In the struggle to depress junior researcher wages for teaching, universities employ various tactics which I will not go into here; but one of the tactics they use is finding TAs who would work for even less, such as later-year undergraduates. A similar phenomenon occurs with teaching courses proper, i.e. giving lectures and administering a course: Universities sometimes try to put later-year M.Sc./ Ph.D. candidates in those positions instead of untenured researchers or teachers who "just" work for the university and cost more.
Now, if it were the case that such teaching were to be undertaken with an extra-ordinary conferment of relevant status, and full pay and benefits (or perhaps - a little higher in recognition of an exceptional record of the person in question) - that would be... well, fine, I suppose. But what happens in practice is that this is used to "juniorize" teaching more widely, with much reduced pay. And that also means lower teaching quality - on average and speaking in rough terms.
Now, if undergraduate students were to somehow be made willing to teach for free, and the university could get away with that legally (which it might not by the way) - managements would have a field day.
So, please do not undermine graduate teachers/researchers' livelihood. If you're interesting in TAing - inquire about doing it for pay. Otherwise, if consider initiatives such as:
- Extra-curricular extra-practice sessions, e.g. organized via the student union, if the course is particularly tricky to get through.
- Investing in creating some video content to help students with some of the tougher concepts in a particular subject