Verified answer

I just looked in my Death Note 13th "How To Read" manga. It has all information on the characters (i.e height, blood type, weight) and I could not find anything about the IQs for the characters. I do not believe you will find anything about that.

EDIT:

There is a chart that shows how good they are in intelligence and other skills. This is the order of intelligence by that chart:

1. Light

2. Near

3. L

4. Mello

In truth the chart shows Near and Light as equals in the way of intelligence.

EDIT 2:

In the 13th "How To Read" Death Note manga. You'll have to buy it to see the chart. I did look around online, however, it doesn't show it anywhere.