Just about all aero structures have critical vibration frequency and amplitude limits. Engine cowling is no different.
It is primarily designed for noise suppression, with strength considerations for ramp rash (nose cowl) and anti-flutter properties (clamshell & nose cowling). Modern engine cowlings are not considered primary protective structures from high energy engine parts. That's the purpose of the engine and fan cases.
While obvious, it is useful to recall that blade out test rigs are designed primarily to evaluate the disk, fan, and fan case strength and performance. The inlet bell is a multi-purpose structure that has no bearing on the blade test data, by design, outside of the purpose of containing the high energy fragments.