Yeah I wish they had some budget to shoot an extra scene to show that attacking head on would've ended up with thousands of dead, which is why Lexa took the diplomatic approach. Remember, she is trying to be a different kind of leader to her people. If anything, it would've worked and she would've at least been remembered for negotiating her people to their freedom from the Mountain Men. Because think about it, if you and the Sky people joined forces and you still had doubts you could win but you had a chance to free your people at the cost of theirs, you would take it.
It's just that no one really expected the Sky People to even take on Mt. Weather after that. Remember, they fought tooth and nail for that alliance. And then you ditch them but then Clarke basically single-handedly wipes out Mount Weather. Of course Lexa looks weak after that. She took her shit and ran and lost out on the glory of destroying the Mountain Man. She looks like a coward compared to Clarke who did the literal impossible.