Is bunch of roses a mass noun?

No, a mass noun (also called an non-count or uncountable noun) is a Word for something that is indivisible into countable units.

The noun 'bunch' is a count noun. The plural form is bunches.

The noun 'roses' is a count noun, the plural form of the singular noun rose.

The noun 'bunch' is functions as a collective noun in the term 'a bunch of roses'.

A collective noun is a Word used to group people or things taken together as one whole in a descriptive way.