Presumably, in a complete Wikipedia, the percentage would be much lower (I believe the current estimates are that ~5% of all humans are currently alive, and I'd guess our existing biographies are more about alive people than that). But how does it compare to other encyclopaedias?
That's a pretty irrelevant number. The %age of (known) notable people alive today is much higher. How many people who lived over 1000 years ago do we know enough about to write an article on (if we still know about them, they're notable enough for me, so it's just a matter of them still being known)? A few hundred, maybe? (most will be rulers of the various countries that have records going back that far, I guess) The number increases as we get nearer to present day, but the proportion of living people that are notable is far higher than the proportion of dead people that are notable (that we know of).