You should aim for an L-cancel success rate of 100%, and in practice should achieve something very close to that. In competitive play, even at mid-level you are expected to practically never miss L-cancels in common situations, and only rarely miss them in awkward situations (ones that involve ambiguity as to whether or not you will land or hit something, and when you will do so). Moves are balanced around their L-cancelled landing lag; therefore, in many situations when not L-cancelled they are extremely punishable, and will also combo much less effectively.
You will improve your consistency with practice. If you play other people who can/will punish you for your missed L-cancels then you will improve much more quickly since you can't be complacent about missing them against those players. Look out for specific moves or situations that give you problems and grind those out.
Note that because of hitlag, there are different timings to L-cancelling (and fastfalling) moves which connect with an opponent (either their hurtbox or their shield) and moves which don't connect; you have to delay your L-cancel when connecting with an opponent because hitlag adds a delay before you hit the ground.