Is a giraffe dumb

"Dumb" means that an animal (or person) is incapable of making sounds to communicate. Whilst giraffes are generally regarded as silent animals, they do make vocalisations in some situations, so they could not be regarded as "dumb".

If the question refers to the intelligence level of giraffes: while they are not among the smartest of animals, they most certainly can learn and adapt their behaviour, which indicates that they are not stupid animals.