Yes just look at the image above the text ads may be good for self-help, science, etc niches and image ads is good for niches like games. This is proven. I have tested a lot of times that this is true, different niches get different rate of CTR. For example, if the person who are looking for freebie and he found your website via Google, and clicked on your site, it is high chance that he will not click on your ads, as his intention is to get freebie.