The (cis)/(trans) nomenclature (or, more universally applicable, (E)/(Z) nomenclature) works fine for isolated and conjugated double bonds. In the case of 2,3-pentadiene, however, where double bonds literally are adjacent to each other, you have a cumulene. As a result, the mutual orientation of two terminal methyl groups is close to perpendicular.
To put things in perspective (literally), build a model with a model kit:*
and apply the nomenclature for axial chirality instead. You assign the methyl groups a higher priority than the hydrogen atoms (CIP rules), then, while looking along atoms (2) and (3), the «rotation» along the sequence 1 -> 2 -> 3 -> 4 is clockwise, thus it is (R)-penta-2,3-diene.
As highlighted by a comment by
@user55119, there is some conceptual overlap between axial chirality and helical chirality. As a result, beside the notation by (R) and (S), the description of cumulenes may rely on descriptors (P, as in plus) and (M, as in minus) instead.
*In case you do not have access to a model kit, you may paste the following SMILES string (a structure description a [dedicated] computer program understands)
CC=[C@@]=CC as input for molcal.org. Don't mind that the angle enclosed by the methyl groups differs from the one you get with a physical model kit (example), focus on the relative orientation.
Moss, G. P. Basic terminology of stereochemistry (IUPAC Recommendations 1996) in Pure & Appl. Chem. 68, 1996, 2193-2222; doi: 10.1351/pac199668122193 (open access).