Most characterisation work on fused deposition modelling-printed lightweight polylactic acid (PLA-LW) focuses on peak load resistance, leaving the pre-fracture deformation regime underexplored. This regime governs stiffness, deformation tolerance, and energy absorption per unit volume, and is decisive for deflection-limited and impact-tolerant lightweight components. In this study, three comparative mechanical indices were extracted from 114 quasi-static tensile records spanning Phase 1 (Cubic; eight temperatures; n = 6 per temperature) and Phase 2 (Cubic, Gyroid, Sub-Cubic; two temperatures and two flow rates; n = 3–6 per condition) at 20% infill ratio: the secant stiffness index E_s, the apparent strain index ε_app, and the specific toughness index U/ρ. Two Gyroid F80/240°C specimens were excluded after stress–strain curve inspection revealed grip-slip artefacts, evidence as early-loading plateaus in which crosshead displacement is taken up by specimen seating and slip at the grips rather than by gauge-section deformation. The remaining three valid specimens at this condition were retained. Phase 1 ANOVA confirmed a highly significant temperature effect on E_s (F(7,40) = 15.13, p < 0.001). Near the foaming onset (210°C), E_s peaked at 117.4 ± 10.7 MPa while ε_app reached its descriptive minimum (F(7,40) = 1.80, p = 0.114 overall; F(6,35) = 1.07, p = 0.400 excluding 190°C). In Phase 2, flow-rate effects on E_s were significant for the Cubic (t(19.4) = 16.37, p < 0.001, d = 6.68) and Sub-Cubic (t(19.4) = 7.25, p < 0.001, d = 2.96). The Gyroid showed no significant flow-rate reversal (t(15.8) = −1.70, p = 0.108); however, both Gyroid flow-rate conditions exhibited a significant temperature sensitivity (F70: p = 0.006; F80: p = 0.005). Deformation-regime indices diverged substantially from strength trends; no single topology ranked highest across all three dimensions simultaneously. Future work should validate these comparative indices using extensometer or digital image correlation measurements.