Yes it can be depending on your site. The important thing to do is test to see if this works or not. Some have found that the careful use of small thumbnails above the ads can increase CTR, however make sure you obey the TOS and do not draw undo attention to your ads. Some have also found to place a banner ad in the middle of the content to be also useful but again test it on your site. Always check with Google if you are unsure about the rules. They are the one paying you.
My feeling is that if you place the advert below the fold, the first impression of the site is much better - and looks much more professional and on-topic. Your readers are more likely to read your page before clicking. So yes, perhpas the CTR is less, but you are provding a more quality user experience. Guess it depends on your motives.