Is a thin silicon case for just the TOP portion good for MacBook Air protection?

Do not use anything (plastic or otherwise) that can snap onto the MacBook's lid if trying to protect it (same goes for the bottom case which you've decided against for good reason). If you must protect it in some way, it will likely be better to use a cling-held type of plastic sheet to prevent potential damage by any adhesives used to hold it onto the top of the lid...& that's the top of the lid only. Not the edges nor anywhere near where the screen begins.

Note that I have no idea of something like this even exists so take that advice with a grain of salt.

Yes defiantly, One with vents on the bottom is fine and recommended also. Silicon has a euphoric texture