1. Anon: Not yet, Anon! Mimikyu might become the ghost equivalent of what are "Hunter Exeggutor" - omnivorous / carnivorous Exeggutor that have become actively human-aggressive as an entire, discrete subspecies (these are Exeggutor that target human beings on purpose. They use coordinated stalking tactics, hide in plain sight, and engage in psychological warfare. They strike to maim and can't be negotiated with.) Mimikyu would likely be very similar since camouflage / mimicry is basically its defining feature. They'll stalk both children and adults, I'm thinking. I'd have to spend more time on the species, but overall I'm getting a sort of "dangerous infestation" feeling from them. If you have one in your home, chances are you have several more. They'll make noise to keep you awake, startle you, and overall attempt to wear you down. For what purpose? I'm not sure; maybe they're carnivorous, or they feed on a specific band of bad vibes.
2. @scheming-pangolin: They do not! Grimmsnarl sits at the top of the food chain in their territories. Their offspring are easier targets for other Pokemon, but an adult Grimmsnarl is not a fight even a fully grown dragon-type wants to engage with.
3. @polygonfighter: No worries! Glad to answer any questions people may have about the setting! In regards to how psychic powers work in the base sense, that's an aspect that I'm not explaining for the time being and leaving it within mass-media base assumptions, because there's a certain level of over-explaining on typing and battle functionality that I want to avoid. Surface-level overviews are about as far as I'm really willing to go, because if I pick apart how one "type" works, I have to do it for the rest, and if the drilling goes too deep it'll create pitfalls that will just topple the whole thing. Kind of like how if you think about how Pokemon works in general a little too hard the whole premise completely collapses under the weight of its own social, ecological, and economical absurdity. As a result: psychic powers are simply psychic powers, rock beats scissors, and if you looked at a ghost in a particular way on your last turn you can hit it with Body Slam on this turn.
In terms of weaknesses for psychic-types against ghost-, bug-, and dark-types, some of the reasoning is more or less trying to insert logic into what Game Freak may have been thinking when creating the weakness and resistance map for these less obvious typing matchups. On the surface, why would a psychic-type be weak to an insect? Taken at face value, there isn't really a reason, so there's some extrapolation and making of assumptions involved. My personal assumption is that the psychic < bug matchup was probably as simple as "bugs can be creepy and can make even put-together people upset". RE: videos online of otherwise very calm or tough people discovering a whole new personal vocal octave when they see a spider or a moth fluttering at them. Also, bugs are simple creatures with brains formed from only a cluster of a handful of neurons. In essence, my assumption is that in terms of the OG game logic, bugs are stronger than psychics because they are simple minded (can't really be influenced by mind control) and creep out psychics (psychological warfare, after a fashion). In terms of the setting, this kind of forms the backbone for psychics having a hard time against bug-types. Bug-types have innate psychic resistance, whether because they are simple minded, have a neural map that confuses psychics trying to batter the brain's ability to weather psionic assault, or some other feature. Bug-types can get a hand up on psychics attack-wise because they already pose a defensive problem for psychics, and many bug-types are just upsetting to psychic-types in general. Many create noises that interfere with or mimic psionic broadcasts that impress pressure upon the brain, causing issues with focus, or they release chemicals that interfere with mental clarity, making a psychic Pokemon less effective in battle.
Ghost-types are similar to bug-types in that they enact psychological warfare by and large by just existing (RE: trying to insert logic into the canon weakness and resistance chart - my assumption is that the logic behind it is that ghosts are creepy and will scare even the smartest person). In the setting, ghost-type Pokemon are synonymous with body horror and things "not being as they should". Their brains and entire bodies are made completely different from standard Pokemon stock. There isn't a Pokemon that exists that wants to fight a ghost (except for dark-type Pokemon and other ghosts, they will eat a ghosts / each other if the opportunity presents itself). Ghost Pokemon are illogical and frighten psychic-types. Ghosts can also reach into a person's brain in a manner not dissimilar to a psychic if they are so inclined (see the answer post that mentions "possession"), but the manner in which they do this is unique to them and can do catastrophic damage.
Dark-types, judging by how / when they were introduced in canon, is by my assumption a blanket answer to how overpowered psychic-types were in gen 1, and in the project setting this is the logic behind them, as well. Psionics as a feature is present in the human race long before Pokemon become a thing, and psionic resistance becomes something that is industrial-grade by the time the conflict that sparked Pokemon creation reached its mid-point. A whole strain of sub-human fighters to counter psychically-enhanced sub-human regiments became a must, and what was previously a textile-only technology was forcibly warped, reconstructed, and directly appended to the source genome that would be used to create a sub-section of the second generation of obedient militia.