Increasing penetration of Renewable Energy (RE) based Microgrid (MG) introduces either beneficial or detrimental effects on power system stability. Uncertain condition of RE influences the dynamic behaviour of critical modes of power systems which potentially alter damping and increases the risk of oscillatory instability. On the other hand, it also reduces generator and transmission lines stress, enhancing the system stability. More complex MG control also leads to the occurrence of interaction among critical modes. More specific modal interaction in terms of resonance phenomenon potentially emerge when the engaged modes align both in damping and frequency of oscillation. In this paper, impacts of uncertain power injections from the RE based MG on small signal stability are thoroughly investigated using Monte Carlo (MC) simulation. From eigenvalues analysis, it was monitored that under RE uncertainties, the critical modes behaved randomly and unpredictably. However, the cumulative distributions of critical modes indicated that the additional power injections from the MG system introduced a positive impact in enhancing system stability. The presented work further discussed interaction among local, inter-area and control modes of MG due to variation of system parameters using a novel index known as Modal Interaction Index (MII). From several case studies, it is observed that MG contributed to mitigate resonance and modal interaction.