If you just sew a border strip on without measuring and pinning first, you almost always get that ruffle effect. Too much border gets sewn onto the edge.
To solve the problem, measure through the middle of the quilt and cut the border to that length. Fold both the quilt and the border into halves and quarters, and pin generously matching edges, centers, quarters, etc. Only then do you sew, easing the quilt edge if necessary to fit the border.