@cocoa_vrc 調べてみるとimportMultibaseKey()関数でエラーが出ますね。

error: Uncaught (in promise) TypeError: Unsupported key type: 0x22
    throw new TypeError("Unsupported key type: 0x" + code.toString(16));
          ^
    at importMultibaseKey (https://jsr.io/@fedify/fedify/1.4.0/runtime/key.ts:105:11)
    at Function.__fromJsonLd__Multikey__ (https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:18311:15)
    at eventLoopTick (ext:core/01_core.js:177:7)
    at async https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:18151:26
    at async Function.fromJsonLd (https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:18147:12)
    at async Function.__fromJsonLd__Person__ (https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:60794:9)
    at async https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:60622:26
    at async Function.fromJsonLd (https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:60618:12)
    at async Function.__fromJsonLd__Object__ (https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:7579:16)
    at async https://jsr.io/@fedify/fedify/1.4.0/vocab/vocab.ts:7203:26

@cocoa_vrc 参考までに、検証に使われたスクリプトは以下の通りです。

import { Object } from "jsr:@fedify/[email protected]";

await Object.fromJsonLd({
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/v1",
    "https://w3id.org/security/data-integrity/v1",
    "https://www.w3.org/ns/did/v1",
    "https://w3id.org/security/multikey/v1",
    "https://www.w3.org/ns/cid/v1",
  ],
  "type": "Person",
  "preferredUsername": "apsig_dev",
  "id": "https://apsig.amase.cc/actor",
  "inbox": "https://apsig.amase.cc/actor/inbox",
  "outbox": "https://apsig.amase.cc/actor/outbox",
  "assertionMethod": [
    {
      "id": "https://apsig.amase.cc/actor#ed25519-key",
      "type": "Multikey",
      "controller": "https://apsig.amase.cc/actor",
      "publicKeyMultibase": "z3Jtah4FBxr5Xm6bpazSPnCG72fbKHRP15QJtDq2vybvo",
    },
  ],
  "publicKey": {
    "id": "https://apsig.amase.cc/actor#main-key",
    "controller": "https://apsig.amase.cc/actor",
    "publicKeyPem":
      "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuqXNerNeWChF4wMMPj4P\nG8GHxFi1bsk8doRrAHHG5RyEtdKYsy5pHHNl8fHzTU4KaGqUg9x8houo0/9HRJlv\nPhtJ9w/i4j93fXJFrNTPvPdcIdM1ZHwdeaOik9QpDen8X755VcNL0vXXlyTMnRln\nY42idm78mbvcKKeOezDrMt+atRP7/9E9AAvqapO1fEStx0JH3MgYlCcIivZkUGrP\nzPnJs6r+FsbHTobg93jvyzXlZBf/iFg/q56nvfH4GQiit4OUPWH44utN8OLIByBE\n8Ue9Lki3/HSElANyzEQ+WyDl/P2xWIOZ1mJHG+HobNGghc750CPfXPFJ+q8O9X2N\nywIDAQAB\n-----END PUBLIC KEY-----\n",
  },
});

Denoがインストールされた状態で下記のコマンドで実行する事が出来ます。（別に依存関係のインストールは不要）

deno run -A --unstable-temporal スクリプト.ts

お役に立てば幸いです。