I've been noticing a few facial features of characters change, most noticeably Wonder Woman in a few comic book panels. But in some panels there seems to be no change when compared to The New 52, they look exactly as they did in the 52 reboot. Is there a definite change or is it just sloppy artwork ?
Short of things like race, scars, alien features and the like (and even these can be up in the air sometimes!), character's facial appearances are rarely constant, they change with each artist. Some artists specifically model their heroes on celebrities, for example, but as soon as the artist changes, they may look completely different.
So to a certain extent, the answer is yes, because they change all the time.
More fundamentally, the answer is sort of also technically yes, because there are different artists assigned to characters, who will bring their own take, and larger redesigns to go with the new Rebirth, however some of these that include changes to facial appearance won't stick around past the new artist (an artist who drew the old WW might draw her very much the same, facially, even if things like the costume are updated) and others arguably aren't even the same "character" (for example, there's a black Wally West, Kid Flash... but Rebirth started with the return of the older, white, Wally West Flash, and they're apparently keeping both characters and retconning into some convoluted family relationship... but prior to this, from the last reboot, black Wally West was probably considered the same character but with a changed racial background... there's also a Chinese Superman who seems to only have the name).
With specific respect to Wonder Woman, DC did release her design during their press push for Rebirth, including an annotated version:
You'll note that none of the annotations refer to anything facially different between the two versions (and, although the angle is different so it's hard to say for sure, the pre-Rebirth reference image seems to be drawn along the same facial features). So it seems that DC at least doesn't think that Wonder Woman's facial features have changed to any degree worth pointing out or keeping consistent among various artists.
It seems like, obscure exceptions aside, deliberate changes to facial features of the majority of heroes are not intended as part of Rebirth, but rather counts as what you'd call "sloppy artwork." So, to answer your question in the spirit it is intended, and with specific reference to Wonder Woman, the answer seems to be:
It's not unusual for the looks of comic book characters to be redesigned so as to more closely resemble an actor's portrayal in an upcoming movie based on the property. The above picture of Wonder Woman is apparently a prime example, redesigned to be more in line with Gal Gadot's portrayal of the character in Batman v Superman (2016), Wonder Woman (2017) and Justice League (2017). Not only is the 'skirt' restored but the skirt design actually reflects that of the films, plus the sword and the shield are now featured prominently - again in line with the DCEU movies.