Sometimes you just have to be the change you want to see in the world.
I had a similar situation in grad school.
It certainly doesn't have to be a grad student in over their head. It can be an overconfident new professor, a doctorate angry over being denied a professorship, a retiring tenured who has lost interest, or just a professor too busy with other work.
The others are not wrong at all when they say that you don't have to do more, maybe even that it all "evens out".
But you can do more.
And in the end, it's your prerogative. As a graduate assistant, you definitely cannot fairly be expected to do more (indeed some unrealistic folks can send blame your way, but you cannot generally be driven about by fear of unreasonable potentialities).
It's unfortunately a common way in academia within science. We idealize it as this army of passionate professors teaching. But it's actually often people whose love is research being required to teach despite a great lack of interest/skill in education. Or a young professor forced to teach yet another course because the tenured folks don't want to.
And it's unfair on the student. Especially if it's an important major course. The fault may well ultimately belong to the university for not having enough faculty in (though that can take time). But the fault is unfortunately irrelevant to the students end result. They're not going to get what they need.
You shouldn't have to... and you don't have to!
Indeed, you likely won't benefit greatly from it, and it could make life a struggle.
But you're likely to be in a unique position to help. If you don't do it, no one likely does. So if you think you can see and solve problems, don't be scared to step up.
In my own circumstance, the teacher was passionate, just scattered and non-conventional. My co-TA and I were first year grad students who weren't connected to the professor but had a friendly working relationship with him (he was a longtime special topics teacher and staff member, but did not hold a PhD to my knowledge). During the lectures, we saw topics that we were passionate about, that are fundamental, and honestly some of the most interesting in meteorology, being mostly passed over. We'd had great professors in our own undergraduate work (at other universities), and we thought it was important that more be taught on the subjects.
So during the regular meetings with the teacher, we casually noted a few details we'd like to see highlighted, and offered upfront to teach a couple days of the course if wished. He happily welcomed it, and the students benefited.
Because of the wider struggles students were having with some of the other imperfections, we also added extra office hours, tried to grade the homework with plenty of detail, and did a final review pizza session. You're a recent student. You know what can help benefit and you can often have a friendly rapport with them. Use that to your advantage.
And because of your situation, you're also a comrade with the person who is teaching the course. I wouldn't stress as much about decorum or hurt feelings, like I might with a real professor. This grad student may well have gotten this class pushed upon him... or didn't recognize how challenging teaching would be... or hadn't yet learned to say no.
Sit down with him and talk it out. Since you say he knows the reviews are bad, offer to help. If you wish, offer to help him lay out his course structure/notes better, or even offer to teach some if you're available. You can also offer him the true reflection of the course he often won't get otherwise (often students, especially newer ones, are fearful/reverent of a teacher, and uncertain what this is supposed to work like. And so they won't approach the teacher). You can be the reality source he/she really needs.
You can only do so much. You won't change the world. But often in these type of scenarios, there's a not lot others can/will do. If you're passionate about the material, to see it taught better, to see people get the quality you yourself received, then feel free to step up. It's a tough choice, one that indeed could stunt your progress elsewhere, be it towards your thesis(?) or in your personal life. And it's far too often thankless. But in the end, if you know the reasons you're passionate to see, you'll know what you did, and that you did what was caring and gave your best, and that's what will truly matter. Once in a while someone needs to be the one to step up for others.