Not going to comment on whether or not DD is OP, but the stats you're referencing aren't the best way to determine item power.
Items bought later in the game tend to have higher winrates (you're more likely to be winning when you can afford a 3rd slot item) and the pickrate stat on that site is partially influenced by champion popularity (an item that Lee Sin builds will have higher popularity because Lee Sin is in a lot of games)
A better way to compare item power is to compare winrates and pickrates VS. other items in the same slot on the same champion. There's always hiccups though when you factor in things like item price (expensive items are harder to get so they tend to have higher winrates), counter items (how strong should an item be when it is ONLY bought to counter something), and snowball items (how strong should an item be when it is ONLY bought when winning).