So quality artwork was not necessary for popular appeal, as there were a number of strips in those days that were as successful as Raymond's and Foster's. Those other adventure strips tended to feature adequate depictions given the constraints of the size of panels as they appeared in print and the need to crank out artwork at a pace necessary for daily and sometimes daily-plus-Sunday publication. That is, corners had to be cut even though a successful strip allowed the main artist to hire one or more assistants to help out.
But the main reasons for an adventure strip's success were plotting and characters. Readers had to be pulled along by the action, anticipating each day what might happen next. And the characters had to be interesting enough that readers didn't lose interest in them.
An example of this was the long-lived (1933-1973) comic strip Smilin' Jack by Zack Mosley (1906-1993). A detailed appreciation worth reading is here.
Smilin' Jack was an adventure strip featuring airplanes, one of several in the 1930s and later. Mosley included drawings of planes as much as he could, placing a tiny one in the background if he couldn't find an excuse to make it more prominent. His drawings of people were marginal. They were simply done, useful for rapid production and appearance in comparatively small space on newspaper pages. But their anatomy -- especially for shapely women -- was distorted. In later decades he tended to make heads and faces too large compared to the rest of bodies. Perhaps that was due to shrinking publication size and a need to somehow compensate.
The strip had a limited set of consistently-appearing characters. This was true of most adventure comics. But the Smilin' Jack cast might have been a little smaller than average around the end of the '30s. Most prominent were Jack himself, a heavy Polynesian named Fat Stuff (or Fatstuff) and Downwind Jaxon, another pilot who often stole the show from Jack.
As the second link above describes it, Mosley wanted to add a character who was really handsome and more successful attracting women than Jack himself (some of the plots dealt more with love life than flying). But he couldn't draw a really handsome face to his satisfaction. So that character, Downwind, was always shown is a pose where his face was averted (usually) or hidden by an object or a speech balloon (sometimes).
(Aside for non-aviation buffs: the term "downwind" has highly negative implications for pilots. One should, if at all possible, never take off or land downwind -- with the wind blowing the same direction as the airplane. That's because airspeed (the speed at which the craft is encountering the air) is lower than its apparent ground speed. For example, a given plane's stalling speed is 100 miles per hour. If it is on final approach for a downwind landing traveling at 110 MPH ground speed but has a 20 MPH tailwind, its airspeed (the flow of air over the front of the wing) is only 90 MPH. That's below its stalling speed, so the airplane will crash rather than accomplish a normal landing.)
Below are some Smilin' Jack panels taken from the Internet. Click to enlarge.