I wouldn't call it a strong alliance.
Remember that they were just at war with the Sky People just a few days before. And Clarke personally fried alive something like 300 Grounder warriors. Most of the tribes in the Grounder coalition had to be dragged kicking and screaming into the alliance with the Sky People.
We know that the Grounders would benefit greatly from a permanent alliance with the Sky People, but from the perspective of the average Grounder grunt, these are the people who came from the sky, stole some our land, burned one of our villages, committed atrocities against us and slaughtered hundreds of our warriors through underhanded tactics and that we aren't allow to extract vengeance from.