Is cis dipole moment is always greater than trans dipole moment?
No.

The dipole moment of trans-cyclooctene ($\pu{0.82 D}$) is greater than cis-cyclooctene ($\pu{0.43 D}$). 
See On the Molecular Geometry of trans-Cycloöctene, J. Am. Chem. Soc. 1958, 80 (8), 1953–1955.  


  Blomquist, Burge and Sucsy2 have called attention to the fact that while with the cyclodecenes, the cis isomer has the higher density and index of
  refraction as is typical in acyclic systems, with the cyclooctenes, as first noted by Ziegler and Wilms,3 the reverse is true. From these refractive indices, by application of van Arkel’s dipole rule,4 the prediction could be made that the trans-cyclooctene would have a higher dipole moment than the cis. 
  
  [...] 
  
  The dipole moments of the cis and trans isomers of cyclodecene were previously determined,4 and were found to be normal. The conclusion, drawn from the dipole rule, that the trans-cyclooctene would have a higher dipole moment than the cis represents a situation without analogy. 
  
  [...]  
  
  The cis isomer was found to have $\mu_{cis} = \pu{0.43 \pm 0.06 D}$ which may be compared with the value found for cis-cyclodecene of $\pu{0.44 D}$.4 The trans isomer was found to have a dipole moment of $\pu{0.82 \pm  0.03 D}$, compared with the value zero found for trans-cyclodecene.4 


(Van Arkel's rule is: The isomer with the higher dipole moment possesses the higher boiling point, refractive index, and density.)