No, your approach of fetching data from a Core Service's server action in a Core Widget module is valid and adheres to the OutSystems Architecture Canvas rules.

The Core Widget module, as described in the Architecture Canvas and related best practices, is designed to hold reusable UI components (e.g., web blocks) specific to business concepts. It is expected that such components may need to fetch data from the Core Service layer, which provides business services, entities, and business rules. This hierarchical relationship ensures no upward dependencies or cyclical references, following key architecture rules:

1. No upward references: The Core Widget sublayer belongs to the Core Layer and can reference modules in Core Services.
2. Separation of concerns: The Core Widget focuses on reusable widgets and UI logic, while the Core Service provides the required business data and logic for these widgets. This keeps the architecture modular and maintainable.

As long as your Core Service modules encapsulate the business logic and expose public server actions for Core Widgets, your design is sound and compliant.

For further insights on best practices for Core Widget modules and their interaction with other sublayers, check related guidelines in Different OutSystems Applications Architectures and Validation Tools.