I too like aspects of all the Hessian sisters. Their common denominator was their desire to help others and emulating Princess Alice's own life when she came to Darmstadt. And IMO it is not comparable to say Irene would have been better suited to be Empress much less NII wife.
MF and AIII were vying for a dynastic marriage that never came to fruition. Had AIII lived, perhaps together AIII and MF would have come around and not have minded letting NII marry whom he wished. AIII agreed it seems because he knew he was dying. But being relatively young and newly widowed, MF was still in mourning and had to give her position to whomever NII married. Irene or not, the position of Empress was critical. The Russian people would have judged anyone harshly, I think, because they had loved MF and were charmed by her. The new Empress would have to fill that void left by MF. There really is no proof either way that Irene would have done a better job.
She was good at keeping low profile and dealt with her sons' hemophilia in her own way. But her role, a Bsquared said, was not as dynastically important nor were that of her sons. Keeping a low profile is not what being a Russian Empress was all about. As AF tried to do because of her innate shyness and the results of course speak for themselves.