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!