It was all Brian's idea! He had it all in his head, man! And so on

I think Mike's voice just deteriorated, not that nasal sounded better - Night after night of belting out songs and doing your best Jagger impersonations for 2 hours plus could certainly wreck an untrained and not especially powerful voice like Love's, especially as the band grew rowdier throughout the 70's.
Yeah I agree that the passage of time didn't do his voice any favors, and in the early days, the nasal tone wasn't
as annoying as it would later become. I just wish he'd sang like this more often in general - his singing on stuff like "Please Let Me Wonder" (the original version where he does the lead and the finished product where he does the chorus with Brian) and "Kiss Me Baby" is so much more pleasing aesthetically, to my ears anyways. In those days, it didn't seem quite as put-on as it was later.