Kolmogrov complexity is one objective-ish measure of simplicity. The downside is, many hypotheses aren't well formulated enough to even have a vague range of their kolmogrov complexity.
Basically, any theory you can simulate has a kolmogrov complexity, so we can estimate the kolmogrov complexity of Newtonian mechanics, relativity and quantum physics. Relativity has a HIGHER complexity than Newtonian mechanics, but it also explains more things so it has the edge regardless of being more complex.
In comparison, the kolmogrov complexity of your average religion is... harder to estimate, for obvious reasons
