Couldn't be better
could actually mean two things in English - Simple Past tense or present irrealis.
Konnte nicht besser sein
is simple past in German
Könnte nicht besser sein
"Konjunktiv" (irrealis) in the present.
Depends on what was actually meant in the original sentence - I think your example is already wrong in English, however. If you were asked about your holidays, you should probably answer "Couldn't have been better" - After all, the holidays are over now. The proper translation of that would be
Hätte nicht besser sein können
as correctly pointed out in Clayn's answer.