When you set up a Google Display Campaign, although you have selected the interest targeting closely related to your products/services, you will realise that some of your advertisement will be shown on some mobile games/mobile applications.
Usually ads showing on mobile ads will have lower conversion. This is because users on the mobile, they are playing games or using the applications. They are focusing on the games or the apps that they are using. When the advertisement shows up, user might have a chance to accidentally click on the ads. Hence, you will get lots of clicks but do not see much conversion.
You can optimise the campaign by removing the mobile games/mobile apps from your targeting.
There Are TWO Ways To Optimise The Mobile Placement:
1) Manually Exclude One-by-One
You will need to go through the placement report, and exclude those placement that do not convert from your campaign targeting.
Go to your campaign, under the "Display Network" tab >> "Placement" >> for the placement that you want to remove, click on dropdown from status beside the placement and select "Excluded (ad group)".
2) Bulk Exclude From Campaign
You can exclude the mobile placements completely from your campaign targeting.
Go to "+ Targeting" >> Under "Campaign exclusions", you will find a drop down menu "Add Campaign Exclusions", select "Site Category Options".
You will see a section of content optimisation, look for the options "In-game" and "GMob mobile app non-interstitial". Click on the dropdown option, select "Excluded". As long as you see the icon turn into "RED DOTS". Google will not show your ads on mobile app and mobile games.
TIPS: You should always check your "Placement Report" on weekly basis to exclude the placements that cost a lots but do not have generate any conversions.