Professor Echo, the question of star likenesses comes in two parts. As you suggest, many artists aren't good at getting a likeness and some don't even try. Some really good artists are weak on likenesses.
But lack of character likenesses has also been driven by contractual considerations. An obvious example is St. John's Jackie Gleason comic, available on this site. In the first issue's "Honeymooners" story Ralph (Gleason) is the only star whose face we see. His wife Alice is only drawn from behind and his best buddy Norton has his face wrapped in bandages throughout! In the second issue Alice is allowed to show her face but she doesn't look like Audrey Meadows. Norton is unbandaged but drawn from behind. Norton finally shows his face in issue 4 but looks nothing like Art Carney. Obviously lawyers were involved here.
Several decades ago I corresponded with a couple of Dell artists and got a peek behind the scenes. Frank Thorne told me that when he was assigned Disney's "20,000 Leagues Under the Sea" movie adaptation, he received a huge pile of production stills of the sub, the set design, etc., but the film still hadn't been cast. They told him to make up his own characters, which of course didn't remotely resemble the final cast. Alex Toth said that John Wayne refused to allow his likeness to be used in the "Wings of Eagles" adaptation. Dell told him to draw somebody who looked kind of like Wayne but not really. Oddly enough, in Edd Ashe's adaptation of "Rio Bravo" Wayne did look like Wayne. Different agent?
Famously when Alberto Giolitti's Italian art studio got Gold Key's "Star Trek" assignment no one there had seen the show and for reference they had only a few head shots and some pictures of the Enterprise. You can't blame Nevio Zeccara for getting everything wrong, from showing rocket exhaust to drawing the landing party with backpacks (and stylish caps for the ladies).
Link to the book:
My Little Margie 01