Is it possible to be “too” optimistic? Certainly.
Between the two poles of optimism and pessimism there exists a continuum. On one end are the optimists who border on delusion and refuse to face reality. The quote “Denial is not a river in Egypt” comes to mind.
On the other end of the spectrum lie the morbid pessimists, who gripe about anything and everything. It would be fairly safe to say that neither extreme is constructive.