If you got a warning mail from Google Adsense about Policy Violation that looks like this
Publishers may not implement Google ads in a manner that disguises the ads in any way. In order to maintain a good user experience, it is important for publishers to clearly distinguish Google ads from site content. This includes, but is not limited to: site layouts in which the ads push content below the fold on mobile devices; placing 300x250 or larger ad units above the fold on a mobile optimized site.
Then you must have violated Adsense policies by placing a large Ad Unit (including responsive Ad) above your main content on your mobile optimized site.
Doing this causes the advert to cover the entire page, then the user has to scroll downwards before seeing the main site content.
An example of the violation is shown below.
The Ad disguises itself as a site content, and it is very important a user can distinguish Google ads from site content
Its better to put Larger Ad units below the fold
Here is the Perfect way to display your Ads on a Mobile optimized site
Note: Placing a Responsive Ad unit above the fold on a mobile optimized site is an Adsense policy violation
this is because the responsive ad unit when on mobile switches the Ad format to 320X250 which is not allowed above the fold.