Strictly speaking, its neither. Chevrons don't resemble either a sawtooth wave or a sine wave in mathematical terms. The closest is that it is a smoothed (modified) saw tooth form.
Image from compositesworld.com
Though the initial research used a serrated nozzle in saw tooth form, the final commercial application differs from it. However, the chevrons are repeatedly noted as having a saw tooth form, by NASA among others:
One of the most recent noise-reducing technologies shepherded through the research process by NASA and now making a difference on commercial jet engines is chevrons.
Chevrons are the sawtooth pattern seen on the trailing edges of some jet engine nozzles.
So, it's a choice between saw tooth and sine wave, its better to go with the former.
GE's patent for the Chevron describes the shape as triangular with qualifications:
... each of said chevrons being triangular in configuration
... said chevrons having a compound concave contour both axially between said bases and apexes and laterally between said trailing edges..
