Snow Goon
Medium construct, unaligned
Armor Class 18 (natural armor)
Hit Points 33 (6d8 + 6)
Speed 25 ft.
STR DEX CON INT WIS CHA
14 (+2) 11 (+0) 13 (+1) 1 (−5) 3 (−4) 1 (−5)
Damage Immunities poison, psychic
Condition Immunities blinded, charmed, deafened, exhaustion, frightened, paralyzed, petrified, poisoned
Senses blindsight 60 ft. (blind beyond this radius), passive Perception 6
Languages —
Challenge 1 (200 XP)
Antimagic Susceptibility. The snow goon is incapacitated while in the area of an antimagic field. If targeted by dispel magic, the snow goon must succeed on a Constitution saving throw against the caster’s spell save DC or fall unconscious for 1 minute.
False Appearance. While the snow goon remains motionless, it is indistinguishable from a normal snowman.
Actions
Multiattack. The snow goon makes two melee attacks.
Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.
So this is what I meant when basing it off the animated armor. Start there and keep the ability scores and other parts of the stat block the same to start with. This creature is a CR 1. The modifications you made to its strength score technically drop its to hit bonus to +2 with 3 points of damage per hit. It also makes the snow goon even weaker to grappling and the like. And all the other parts of the stat block are still useful to a snow goon. It should still be immune to poison and psychic damage for instance while having the blindsight limitation (or that can be changed to regular vision).
So now looking at some of the changes you added:
AC 15
Damage Immunities: +cold
Damage Vulnerabilities: +fire
Adding a vulnerability does not change CR calculations, but adding a third immunity puts it at the point where it gets a bonus to effective hp for CR calculations. This is offset by the drop to AC 15 to keep the creature at CR 1. It would also work to have AC 16.
In the comics, all of the snow goons' creation of additional goons takes time. This could arguably be better served as story points rather than combat actions that take six seconds. The Mutation and Growth actions mean that it is skipping a round of combat. Rather than building these into the stat block, let the story work with them. A snow goon that had an additional arm and a second head might have more hp, an extra attack, be CR 2 or 3, and be a potential "boss." Also, the healing option adds 16 hp to a fallen goon which translates to 32 effective hp and turns the snow goon into a CR 2 creature. All of the special snow goon stuff could be done in 6 seconds because, "magic," but it alters the difficulty of the encounter. To keep the snow goons from infinitely duplicating, mayber the magic that created them can only control a maximum number. When some are destroyed, the remaining snow goons work to replace their fallen comrades.
So for a party of 3rd-level party with 4 PCs against CR 1 snow goons, 1 would be an easy encounter, 2 would be Medium, 3 would be Hard, and 4 would be Deadly. A CR 2 snow goon would be 1 for an Easy encounter, 2 for Hard, and 3 for Deadly.
Some of the other parts of the stat block seem a bit counterintuitive or are not how 5e monstersare usually made (not that its a bad thing; plenty of my custom monsters do things differently):
*3 attacks that do less damage versus 2: More dice are being rolled and calculated, which takes more time, which slows things down. The DPR is also 1 less for your version than the standard animated armor.
*Snowball: this doesn't do much damage (hurts DPR), hurts the monster, is not part of multiattack (hurts DPR), is limited per day. The snow goon, like a giant, should be able to just pull its missile attack from the environment since it lives in snow. Changing the multiattack to just making two attacks would allow the snowballs to be used as needed and not hurt the DPR and CR calculation.
*Resistance to slashing and piercing damage: I can see an arrow popping in and not doing much, but a sword could slice a whole bunch of snow goon off. Ultimately, I would drop weapon resistances. Alternatively, maybe keep piercing resistance but add a vulnerability to bludgeoning?
*Bonus to stealth: Creatures usually do not get a flat bonus to stealth if I remember correctly. They usually receive advantage when in a favorable terrain.
*Snow stride: Usually creatures that walk in icy environments ignore difficult terrain versus getting a bonus to speed.
If you want the snow goon to have problems in warm environments, add a trait that damages it when it is in an environment where snow would melt quickly.
So this would be my suggestion for a snow goon. It doesn't have as much of the special stuff, but I would suggest putting that into the story rather than combat:
Snow Goon
Medium construct, unaligned
Armor Class 16 (natural armor)
Hit Points 33 (6d8 + 6)
Speed 25 ft.
STR DEX CON INT WIS CHA
14 (+2) 11 (+0) 13 (+1) 1 (−5) 3 (−4) 1 (−5)
Skills. Stealth +2
Damage Immunities cold, poison, psychic
Damage Vulnerabilities fire
Condition Immunities blinded, charmed, deafened, exhaustion, frightened, paralyzed, petrified, poisoned
Senses blindsight 60 ft. (blind beyond this radius), passive Perception 6
Languages —
Challenge 1 (200 XP)
Antimagic Susceptibility. The snow goon is incapacitated while in the area of an antimagic field. If targeted by dispel magic, the snow goon must succeed on a Constitution saving throw against the caster’s spell save DC or fall unconscious for 1 minute.
False Appearance. While the snow goon remains motionless, it is indistinguishable from a normal snowman.
Frozen Creature. The snow goon loses 3 (1d6) hit points when it starts its turn in an environment that would rapidly melt snow.
Ice Walk. The snow goon can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn’t cost it extra moment.
Snow Camouflage. The snow goon has advantage on Dexterity (Stealth) checks made to hide in snowy terrain.
Actions
Multiattack. The snow goon makes two attacks.
Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage. Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.
Snowball. Ranged Weapon Attack: +4 to hit, range 20 ft./60 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.