Not primary sources, but the most consistent term I've seen to refer to these is "lattice structure," which unfortunately is a generic structural engineering term.
Everyday Astronaut's writeup is the longest:
Another design consideration is due to the blunt nose of the Starliner you’ll see these little lattice structures. The Starliner was designed to be as stable as possible for reentry, which means a short and stout design. The lattice structure helps diffuse the airflow over the vehicle, helping to make sure there are no shock waves or any inadvertent pressure areas on the lower portion of the vehicle on the scent [sic, probably ascent]. Especially since the rocket actually tapers down to the skinny centaur upper stage. They also added an aerodynamic skirt to ensure smooth airflow.
This Spaceflight Insider page includes an infographic, credited to Derek Richardson, Spaceflight Insider, and Orbital Velocity, that mentions the lattice structure as well and credits it with diffusing "inadvertent pressure waves." They credit this Boeing launch press kit as their primary source, but I haven't found a version in The Wayback Machine that includes any information about the lattice structure. I'm going to reproduce the image here in case of link rot.
My understanding, based on these sources and a compressible flow course I took 15 years ago, is that these parts are for managing shockwaves during the trans-sonic regime. Once the vehicle is fully supersonic the taper from the Starliner's diameter down to the Centaur's diameter is protected by the bow shock, but just before the entire stack reaches Mach 1 there's a chance shocks will form there and cause pressure spikes, leading to vibration and stress. This perforated lattice structure trips the shocks upstream, next to the large flat surface of the service module.
Searching for "Starliner shock management" got me this excellent Twitter thread by Chris Combs which digs into the history of aerodynamic studies of "Hammerhead launch vehicles," including more nuanced/correct dissection of the aerodyanmics than mine:
At transonic speeds (M ~0.7-1.3) you get a shock structure that can form just downstream of your capsule. This shock interacts with the growing boundary layer/separated flow & oscillates rapidly. This produces large pressure loads
Combs also links to a tweet from Tory Bruno, which is frequently one of the best publicly-available primary sources on Starliner or older ULA matters:
Those are for Aerodynamic management. Helps to control the flow around and past the skirt. The skirt is there to manage the location of the shock attach point on Centaur which would otherwise be high up and asymmetric