I think you meant to say "in German".
After all, it is also perfectly possible to get a bad dinner in Austria or Switzerland. ;)
If the context makes it clear you are talking about dinner, you would just say "das Essen".
"Das Essen schmeckt furchtbar."
This is as direct as in English. It's very blunt and almost an insult to the cook.
"Das Essen schmeckt überhaupt nicht gut."
This is still rather direct.
"Das Essen schmeckt mir nicht"
This way you say it does not match your personal taste. This is acceptable to say to the cook if you know each other well enough.
"Mir schmeckt das Essen nicht so"
By placing "mir" in front, you emphasise even more that it's because of your personal taste. "Nicht so" is like "not so much", so it is weaker than "nicht".
If you say it right at the table, I would only recommend the very last option.