Strong disorder in superconducting nanofilms enhances kinetic inductance while simultaneously weakening phase rigidity, creating a competition that strongly shapes their low-temperature electrodynamics. To investigate this interplay, we study reactively sputtered NbN films with thicknesses ranging from 25 nm down to 2.8 nm, achieving sheet kinetic inductances as high as 300 pH/□ while maintaining Tc ≈ 7.8 K. In the thinnest film, DC transport reveals a resistive tail between the mean-field transition temperature, T_C0 = 7.41 K, extracted from Aslamazov-Larkin and Maki-Thompson fluctuation conductivity, and a BKT transition at T_BKT = 6.42 K, demonstrating the dominance of phase fluctuations. To connect this behavior to microwave electrodynamics, we measure CPW resonators, where they operate deep in the kinetic-inductance-dominated regime (α ≈ 0.99), so that δf/f directly probes the superfluid stiffness. In this regime, all films exhibit a low-temperature power-law suppression, δf/f ∝ (T/T₀)^b with b ≈ 2.3-3.2 below 0.35 Tc, inconsistent with Mattis-Bardeen activation. This anomalous response becomes progressively weaker with increasing thickness, revealing a continuous crossover to conventional BCS behavior above T*. We find that this crossover is governed by the ratio of superfluid stiffness to pairing energy, Θ(0)/T₀, which spans ~3 to ~48 across the series and emerges as the key parameter separating phase-fluctuation-dominated from gap-limited electrodynamics. While strong dielectric loss is confined to the ultrathin limit, thicker films reach Qi ≈ 10^5-10^6 with weak power dependence, establishing NbN as a practical platform for compact high-impedance quantum circuits.