"Nested Universe Theory" isn't a standardized term, because on its own, it could mean a half-dozen different things with completely different implications. It all depends on the nature of the nesting.
You need to be more specific. Here are the common ways this is portrayed:
1. Physical Containment (Russian Doll Universes): One 3-D universe is simply physically located inside another, much larger 3-D universe. This doesn't grant a higher tier. Destroying the "outer" universe would just be a High 3-A feat, as you're just destroying a larger infinite 3-D space. Being bigger in size doesn't mean you're higher-dimensional. An example of this is the end of Men in Black
2. Higher-Dimensional Embedding: The "inner" universe is a lower-dimensional slice or membrane of a "higher" universe. For example, a 3-D universe existing as a part of a larger 4-D reality. This is genuine Higher-Dimensional Existence. Destroying the 4-D structure would be a Low 2-C feat
3. Reality > Fiction Transcendence: The "outer" universe views the "inner" universe as fiction, like a book, a dream, or a video game. This is a qualitative superiority, not a dimensional one. A character in the "real" world would be 1-A in relation to the fictional one
4. Conceptual/Ontological Nesting: The "outer" realm is a more fundamental, abstract, or conceptual plane of existence that governs the "inner" physical universe. For example, a realm of platonic forms from which the physical world is just a shadow. This is also a qualitative superiority and would be tiered at 1-A