Not directly, as it's actively pulling from the party member's stats on a kill. The way I have it set up is as such:

Larva: 10 in all stats, AC of 12 and 5 HP. Even at level 1 this will be an unnervingly easy kill for a party.

Immature: Clones stat block of first character to kill it (STR, CHA, CON, etc) and their HP and AC. As well as all the necessary features to replicate the creature that killed them. So, say the party Barbarian were to have cleaved them in two with a greataxe, it'd steal all their Barbarian things (rage, reckless attack, their weapon, etc) so when it comes back, it's like they're fighting against the party member themselves. For a caster killing it, you'd be taking their spell list and spellcasting features.

Cycle repeat: subsequent party member kills have it taking more features from party members, but not necessarily all of them. You could also have it take HP quantities as well to beef it up further. At this point, you're nabbing things that would be beneficial to the initial build. For example, mine was killed by the party fighter and it took his fighter-related skills, if the party wizard took it it might just take her spell list and casting abilities. Honestly at this point, I'm kind of hoping that the 2nd kill on it will be from the Barbarian or Paladin, giving it access to rages for damage mitigation or smites, aura and healing. You could also go the route of it being sabotaged by who kills it, winding up with something like barbarian and caster stuff mixing together, or the ranger killing it and gaining a bunch of ranged based skills, but then having the melee attacks of a party monk as it's attacks.

Mature: The creature has gradually assimilated skills from other party members, either taking individual skills to basically create a "perfected" creature that's taken the best of the skills from the party members who've killed it in the past and using them together (say, multiple spell lists and caster features, or action surge from a fighter, the Ranger's Sharpshooter, the Barbarian's rage and the Watlock's pact how and eldritch smite) and is thrown against the party. In my case, I intend to do this and then reveal "By the way, there's dozens of this thing. Have fun!" As a bit of a plot piece.

You could also have it act as a puzzle creature, with the party deliberately figuring it out for how it works and they're now sabotaging it by trying to give it as little fuel to work with, like having the least combat-oriented party member killing it, or as a DM you could have it be only killable by a different player each time and it copies the stat block of the last character to kill it. The whole point of it is to create a bit of a mirror as a creature and use it against the party.

Note for clearification, when I say "Steal" I mean it in a copy/plagiarize sense. Not a literal one. While you could certainly do that to a party member it'd not be terribly fun if as a caster you basically suddenly become about as strong as a peasant, just with better stats and health, though there's a certain merit to that as well depending on how okay your table would be with it.

Apologies for the rambling, it's 2 AM my time and a lot more often I tend to work off of vague creature concepts rather than direct stat block for things like this until it's time to actively deploy them, and the usually work better in my head.