The duplicates aren't creatures
Per the mirror image spell description:
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.
Nothing in the spell description says the duplicates are creatures - they're just illusory copies of you.
...But attacks can always be redirected to them regardless
The description continues (emphasis mine):
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.
Anytime you're targeted by an attack while mirror image is active on you, the spell can redirect the attack to one of your duplicates. The spell makes no exceptions regarding whether the attack can only target creatures.
As the intro to the PHB/basic rules states:
Specific Beats General
This book contains rules, especially in parts 2 and 3, that govern how the game plays. That said, many racial traits, class features, spells, magic items, monster abilities, and other game elements break the general rules in some way, creating an exception to how the rest of the game works. Remember this: If a specific rule contradicts a general rule, the specific rule wins.
It's because "specific beats general" that any attack that targets you, even if it normally can only target a creature, can be redirected to a duplicate as well.
The duplicates can't be intentionally targeted, either
The illusory duplicates can't be intentionally targeted separately from you. As the first quoted pair of sentences state, it's impossible to track which image is real because they move with you and mimic your actions. Thus, anything that would attempt to target your duplicate would necessarily have to be trying to target you - which of the "duplicates" actually turns out to be you is determined randomly by rolling a d20.
Anything that's not an attack doesn't interact with the duplicates
A later part of the spell description says (emphasis mine):
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.
As the spell itself states, the illusory duplicates have no mechanical interaction with things that aren't attacks - whether it's a spell requiring a saving throw, a non-attack monster ability, a class feature, or something else. Unless it's an attack, it doesn't interact with the spell at all.
(Note: grapples/shoves are "special melee attacks", so they can be redirected to the duplicates - but since they don't have attack rolls, they can't "hit" the duplicate and simply fail if redirected to a duplicate.)