I am not a big DiMaggio fan, but there are a few things to keep in mind.
1. Ballparks were bigger. He was arguably the greatest defenisve center fielder ever, and he had to cover a lot more ground than the Andruw Joneses and Jim Edmunds of the world had to cover.
2. He would have hit more homeruns. Yankee Stadium was a hard park to hit homeruns in for a right handed batter. Yes, Fenway hasn't changed, but with some of the ohter bandboxes DiMaggio would have hit for more power.
3. He would have stayed healthier. World War II notwithstanding, modern medicine would have taken his perpetual foot problems in the last few years of his career and cleared them up.
That being said, I do not put him above Mays, Aaron, or Mantle on the list of greatest oufielder ever. But he'd be great these days.