Slightly bitter tasting zucchini is not bad and can safely be eaten.
However if the zucchini is really bad bitter tasting then the zucchini should not be eaten as it could cause issues such as gastrointestinal upsets, stomach cramping and diarrhea.
It's not gonna harm you actually or poison you to eat really bitter zucchini but it's not good for your stomach.
And you'll likely wish you never eaten the bitter zucchini.
It's normal for some zucchini to be slightly bitter but it shouldn't be overly bitter.
The zucchini becomes bitter tasting because of the naturally occurring cucurbitacins that only exist in small amounts in the zucchini, but if large amounts exist in the zucchini, it causes an extremely bitter taste.