'It is too hot' could be translated in an other context to 'on liian kuumaa' and it would then mean that the weather or the inside temperature is too hot.
Here of course it is the coffee that is too hot.
If the weather/temperature is hot, you have to use the nominative: "On liian kuuma."
Why kuumaa and not kumma? If we don't know what "se" refers to, it could be countable or uncountable, right?
Kumma = weird; but both "kuuma" and "kuumaa" should definitely be accepted. You can report that.