Yes, you can avoid fall damage using Misty Step by casting it on the round you would have crashed into the ground.
If you fall for 1 round and within the next round successfully cast Misty Step before hitting the ground, you would be able to avoid taking fall damage according to the top answers of the related questions V2Blast linked.
You wouldn't be able to instantly cast Misty Step right away because the farthest you can teleport is 30 feet and you would still be in the midst of falling the last 500 feet. Since bonus actions can't be Readied, but are only cast on your turn, you can't cast Misty Step as a reaction.
According to the following tweet, a bonus action cannot be readied to be taken on a different turn. However, if you are the only one falling and your turn lasts for the duration of the round, you could reasonably wait until the end of the round to cast Misty Step to avoid the fall damage (emphasis in tweet mine).
A bonus action can be taken only on your turn. You, therefore, can't use Ready with a bonus action. #DnD
Ruling for a DM.
Your DM could have you make an intelligence or wisdom check to see if you judge correctly how far away the ground is when you cast the spell, but that's merely a suggestion.