The elements may be poor quality because the Element Size was left at Default. Try setting that to 10 mm or smaller.
Also, try to set the Mesh Metric for Skewness and set the Smoothing to a High setting. By setting a target value of skewness, you can push the mesher to improve this metric. Please show the statistics for the skewness of the elements.
It seems there is a defect in the geometry in the top left and bottom right corners. Turn on the Close Vertex display marker to see if there is an unwanted extra vertex in those corners and if so, repair them in SpaceClaim.