Either I'm silly, or something doesn't seem to work properly as it crashes on startup. Is there any particular reason for crash or coremod is absent? If so, which coremod I need to install?

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[19:13:42] [main/WARN] [FML]: The coremod Aqua Acrobatics Transformer (com.fuzs.aquaacrobatics.core.AquaAcrobaticsCore) is not signed!
[19:13:42] [main/ERROR] [Aqua Acrobatics Transformer]: No instance of Mixin framework detected. Unable to proceed load.
java.lang.ClassNotFoundException: org.spongepowered.asm.launch.MixinTweaker
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51]
at java.lang.Class.forName(Class.java:264) ~[?:1.8.0_51]
at com.fuzs.aquaacrobatics.core.AquaAcrobaticsCore.(AquaAcrobaticsCore.java:36) [AquaAcrobatics-1.5.2.jar:?]
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_51]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) [?:1.8.0_51]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) [?:1.8.0_51]
at java.lang.reflect.Constructor.newInstance(Constructor.java:422) [?:1.8.0_51]
at java.lang.Class.newInstance(Class.java:442) [?:1.8.0_51]
at net.minecraftforge.fml.relauncher.CoreModManager.loadCoreMod(CoreModManager.java:594) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.relauncher.CoreModManager.discoverCoreMods(CoreModManager.java:451) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.relauncher.CoreModManager.handleLaunch(CoreModManager.java:265) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.relauncher.FMLLaunchHandler.setupHome(FMLLaunchHandler.java:107) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.relauncher.FMLLaunchHandler.setupClient(FMLLaunchHandler.java:81) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.relauncher.FMLLaunchHandler.configureForClientLaunch(FMLLaunchHandler.java:42) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraftforge.fml.common.launcher.FMLTweaker.injectIntoClassLoader(FMLTweaker.java:141) [forge-1.12.2-14.23.5.2857.jar:?]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:115) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: java.lang.NullPointerException
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
... 19 more

The same JAR works on 1.12. https://www.curseforge.com/minecraft/mc-mods/mixinbootstrap/files/3437402

Install MixinBootstrap (or another mod which provides a Mixin implementation).

MixinBootstrap, as I saw it, only for 1.15+, there is no version for 1.12. May I have links instead?

Oh, right. For some reason website did not show this to me. Will test it.

In the meantime, this mod also seems to work in case first one fails. https://ru-minecraft.ru/mody-minecraft/68597-mixin.html. Found it... after some searching.

Seems to work, after hitting it with a wrench a good amount of times. For now.

For anyone using vanillafix and reading this, try next:
try renaming the vanilla fix jar "_aaaVanillaFix-1.0.10-150.jar". If it crashed before, then it means you have a problem with mixin's and another mod is loading an incompatible mixin before vanilla fix (which needs a newer mixin) can be loaded. Force vanillafix to load first by renaming it (it loads in alphabetical order).

Glad you fixed it!