TL;DR the BG3 implementation of mirror image is both numerically worse, and also feels less rewarding to use since you lose one of the mirror images any time an enemy misses you - where often times those attacks were never going to hurt you anyway.
Lets go over the spell descriptions:
BG3 mirror image: Create 3 illusory duplicates of yourself to distract attackers. Each one increases your AC by 3. Whenever you successfully evade an attack one of the duplicates disappears.
Original (5e) mirror image: Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your Actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates. Each time a creature Targets you with an Attack during the spell's Duration, roll a d20 to determine whether the Attack instead Targets one of your duplicates. If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.A duplicate's AC equals 10 + your Dexterity modifier. If an Attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an Attack that hits it. It ignores all other damage and Effects. The spell ends when all three duplicates are destroyed. A creature is unaffected by this spell if it can't see, if it relies on Senses other than sight, such as Blindsight, or if it can perceive illusions as false, as with Truesight.
So whenever an enemy rolls an attack that's lower than your normal AC, you lose one of your duplicates fro mthe BG3 mirror image, with no gain. If the same thing happened with the original 5e mirror image, often your duplicate will also dodge the attack, if the attack is redirected to it. If the attack isn't redirected to the original 5e mirror image, you don't lose it. So the odds of you wasting a mirror image in the original 5e version are quite small. Whereas in BG3 the odds of wasting it can be 50% or more on any attack.
Also with the 5e version it is much more obvious when you are avoiding damage thanks to the mirror image. In BG3 you have to hover over the roll in the combat log and then check if it was higher than your regular AC before mirror image to see if the mirror image actually did you any good there or not.
If an enemy has +7 to hit, and you have 18 AC (breastplate and shield for a trickery domain cleric, or 20 dex + mage armour for a wizard/sorc) then the BG3 mirror image will only prevent about 35-40% as much damage as the 5e mirror image will prevent, per spell slot used.
The Original 5e mirror image also has a good chance of absorbing crits, whereas in BG3 crits ignore the mirror image.
Suggestion for improvement:
I'm assuming that the reason this was changed was becaue the 5e version was too wordy, and you wanted to simplify it. I don't think the huge nerf was intended seeing as so far most changes to 5e I've seen have been buffs in some way or another. So my suggestion is to change the spell to only consume one of your duplicates, when the extra AC provided by the duplicates is what prevents the hit. E.g. If you have 3 duplicates and the enemy missed you by less than 9 remove the duplicate. If it missed you by mroe than 9, you keep all your duplicates. That should be easy enough to program, and then players will know that they dodged a bulllet whenever a mirror image disappears, rather than feeling that it was wasted.
