Almost always this sentence is being used in general and in specific is being used something like "se (it) mämmi" or "tuo (that) mämmi". BUT in certain cases this would also mean specific mämmi. For example in a restaurant you could ask about their food and someone could answer that the mämmi is good -> "Mämmi on hyvää" when it means that specific mämmi. In that case it is clear that it is that certain mämmi so there is no need to say "this mämmi", "that mämmi" or "their mämmi". So yes, it could mean both in general or specific mämmi.
Yes, that is incorrect. A book, a friend, a car, a trip or something like that can be good as "hyvä", but when you talk about the taste, texture or some other feature of something like food or beverage you use the partitive form like "ruoka on hyvää" (food is good/tasty), "liha on kuivaa" (meat is dry), "kahvi on ruskeaa" (coffee is brown), "kiehuva vesi on kuumaa" (boiling water is hot) etc.