It is never a good idea to say that you are not happy. Employers want to hire happy people into their roles, not unhappy ones.

If there are business reasons for you to be actively looking, then provided those reasons are not secret, it is OK to mention that you see softness in your current company's future, that there are structural changes that may affect your job, etc. This shows you have a higher understanding of the business, above your own role or department.