The Jets did very well, but let's not forget that Williams is excellent, and it's a long shot that they will select or sign someone 70% as good. Gardner appears to have regressed somewhat, but he might still be among the top 15%-20% of all cornerbacks, and he'll need to be replaced with a draft pick or a signing.
This is a good trade for picks and clearing salary, but the Jets have a LOT of holes to fill, including QB, and the grade on these moves may not be known for a few years.