You could do that, but it will not be a smooth representation of the inlet/outlet.
You can look at this in 2D by trying to mark a circle on a unit square mesh.
See for instance Plotting subdomains/internal boundaries - #2 by dokken
or
https://github.com/FEniCS/dolfinx/blob/main/python/demo/demo_pyvista.py#L95
or
Defining subdomains for different materials — FEniCSx tutorial