No. If you do that, you'll likely receive a warning about screen-filling ads above the fold on mobile devices. Adsense are red hot on this at the moment.

As Trebuchet said, the biggest responsive ad unit you can use above the fold is 300 x 100.

What do you mean by "on top of the mobile skin"?

I mean my website mobile design .

I saw many website puting Adsense / Ad exchange 300x250 on the top instead of the 300x100 .

so It is weird !

I saw many website puting Adsense / Ad exchange 300x250 on the top

I did that during my visual testing a couple of years ago, it was a horrible experience and immediately removed however I have tried the resposive ad and that looks fine.

I saw many website puting Adsense / Ad exchange 300x250 on the top instead of the 300x100 .

Doesn't mean you should do it. AdSense may just have not gotten around to finding and warning them yet. Or they might be doing pump and dump. If your business plan is for sustainable income, don't pay attention to what others are getting away with, pay attention to the rules.

Here it is

Displaying ads above the fold is not a violation as long as there is enough content above the fold for the users to read. We don't allow site layouts that push all content below the fold so that the only visible content is ads. Implementations like this make it hard for users to distinguish between content and Google ads.

Is placing a 300x250 ad unit on top of a high-end mobile optimized page considered a policy violation?
Yes, this would be considered a policy violation as it falls under our ad placement policies for site layout that pushes content below the fold. This implementation would take up too much space on a mobile optimized site's first view screen with ads and provides a poor experience to users. Always try to think of the users experience on your site - this will help ensure that users continue to visit.

I've received two warnings for this practice within the last month. So don't do it, you'll get a site warning.

Second that, they're enforcing mobile ads above the fold at the moment. It seems to be systematic, I have received warnings for sites that generated very little traffic / pageviews for some of my older abandoned sites with zero to none maintenance. (100 pvs a day)

I got a warning about this today and took the 300 x 250 ad off right away. I appreciated the warning and not getting an immediate ban.

I just moved my 300x250 down another paragraph into the text, problem solved.

Possibly it's clear from this thread but any portion of a 300 X 250 that shows above the fold is an Adsense violation. I got called on this. The example page they used as an example on my site contained a 300 X 250 of which only about 20% showed above the fold.

Funny thing is if you search Google on mobile fo a popular search. You get four ads, then a map with more ads, and finally about three screens down search results. To most people the ads look like search results with the tiny little (Ad) label.

Do as we say not as we do

You are not allowed to put a 300x250 ad above the fold on a mobile layout. I use a responsive layout where I have CSS that shows a 300x250 for desktop and then 300x100 on mobile. The reason is, it pushes the page content down the page so majority of the mobile page is an Adsense ad which is against their TOS. They will get you for it, they got me for it. You can no longer get away with using the old desktop ad units for mobile sites.

Its just easier to use a responsive ad using media queries via CSS to resize the ad based on the screen size because they will also ding you on overlapping ads that go off the screen. I forgot I had a 728 static ad and got dinged for overlapping ads on mobile site.