Background/Objectives: The association of body mass index (BMI) with outcomes after intensive care unit (ICU) admission remains uncertain. We examined ICU mortality and early biomarker patterns in adults with COVID-19. Methods: This single-center retrospective secondary analysis included 201 patients. BMI was analyzed per 5 kg/m² using logistic regression for mortality and generalized estimating equations for biomarkers, adjusted for age, sex, and calendar time. Six inflammatory and coagulation biomarkers were averaged within four consecutive 24 h intervals covering the first 96 h after ICU admission. False-discovery-rate correction and missingness sensitivity analyses were performed. Results: Eighty patients died (39.8%). The adjusted mortality odds ratio was 1.02 (95% CI 0.82–1.26). Joint tests of interval-specific BMI associations were significant for interleukin-6 (IL-6) and white blood cell count, but not the other four biomarkers under the primary specification. Only IL-6 at 24–48 h remained significant after correction across 24 contrasts (−35.9% per 5 kg/m² higher BMI; 95% CI −49.4 to −18.8; q = 0.006), based on 40/201 patients. Conclusions: Within this selected ICU cohort, BMI showed no clear adjusted mortality association and biomarker findings were heterogeneous. The isolated inverse IL-6 association remains exploratory because of sparse measurements and potentially informative missingness and requires independent confirmation with standardized sampling.