She's literally a sitting US representative for Michigan.

So is Jim Jordan, the ultra-MAGA ideologue from Ohio. (He may not even be the most extreme, he's just the one that first came to mind.) If being a representative automatically makes you mainstream, they both are. And at that point, "mainstream" is more like "main river" or "main ocean" - it's so broad that it's meaningless.

Tlaib is one of the four most-extreme-left members of the House. I claim that that makes her not mainstream, for any reasonable definition. She sure is not in the mainstream of the House!

She's mainstream, just because you don't agree with her doesn't mean she's not. Trump is also mainstream, I don't agree with him but he is. We're not talking about podcasters or people xeroxing newsletters, these are elected officials.