Hi, that looks already very good! Just two things:

• This looks very similar to how Node would print such an error. Node, however, would indent every subsequent cause by two more spaces (i.e. two for the first cause, four for the second etc.). Personally I find this helps visually so that it does not look like one huge stack trace. Maybe we want to add this as well? (one easy way to indent is to .split('\n').map(line => indent+line).join('\n'))
• It would be nice to test this output somewhat. Of course we should not test stack traces directly as they contain paths on a user's machine, but we can test stuff in between. Here is how you would do it:

If you do not know how to run tests, look at CONTRIBUTING.md.
Tests for command line output are located in test/cli. Create a new folder in test/cli/samples with the following files:

• main.js: It does not matter a lot, but we need an entry point. E.g. console.log('ok') would be some content that does not produce a warning.
• rollup.config.mjs: We define a config that always throws an error with nested errors, e.g.

  export default {
    input: "main.js",
    plugins: [
      {
        buildStart() {
          throw new Error("Outer error", {
            cause: new Error("Inner error", {
              cause: new Error("Innermost error")
            })
           });
        }
      }
    ]
  };

• _config.js: The test configuration. This would be the typical contents

  const { assertIncludes } = require('../../../utils.js');
  
  module.exports = defineTest({
  	description: 'prints error cause',
  	command: 'rollup --config rollup.config.mjs',
  	// We expect an error and want to make assertions about the output
  	error: () => true,
  	stderr: stderr => {
  		// We just assert the parts of the output that do not change
  		assertIncludes(stderr, '\n[!] (plugin at position 1) Error: Outer error\n    at ');
  		assertIncludes(stderr, '\n  [cause] Error: Inner error\n      at ');
  		assertIncludes(stderr, '\n    [cause] Error: Innermost error\n        at ');
  	}
  });

  If you add solo: true, you can just run this single test, just do not forget to remove this line before committing.