Yes that is correct. It is a common expression when it is very hot to say "it is cooking in here" or "it is boiling".

You may say :"I am toasted " , like toasted bread in the oven

Cooking is purely used to express the application of heat to change the state of food. Because the expression is designed to express only the state of the place you are in, and not that the heat is being used to transform the place, cooking is not an appropriate way to say this.

When a speaker says "something is cooking in here" it can mean he is suspecting/assuming something is happening and is referring it to a third person.

I guess you can describe hot weather with adjective like boiling, baking, roasting but not with cooking. Furthermore these are informal usages.