Not to nitpick but a steep learning curve actually implies that progress accrues quickly after starting (learning/time). A flat learning curve implies difficulty.
To nitpick: almost everyone I've heard use this in casual conversation (who isn't nitpicking) uses "steep learning curve" as in physical analogy, e.g. bicycling to a goal, rather than describing a function plot. A "steep" path is harder going than a "flat" or "shallow" one.
A flat learning curve implies slow learning, which could be because there is little (left) to learn, the need for learning is low, or because it's difficult.
A steep learning curve could be due to having to start with unusual basics which means you accrue knowledge quickly but you can't do much with it yet.