can cassette decks overheat?

I've never seen a deck with vents.  My Teac        V-1050 was worked hard for 20 years totally packed in a cabinet.. Never opened up the case. Good as new.
Throw your Sony's out, I did,  cheap breaking belts.

I have never heard of a cassette deck overheating but if it gets a lot of radiated heat from other equipment it might make the tape in the cassette suffer some damage due to the extreme conditions it is being exposed to.

My Nak CR-5A gets warm even on an open top shelf, but I've also used it years ago as a portable live show recorder in a rack case with other electronics that would get rather toasty without ever having troubles. But, like anything else, you'll want to let it breathe if possible.