Most of her opinions seem to make sense given her background and FWIW Macca's my favorite rock vocalist as well. I generally prefer jazzy singers (Elis Regina, Billie Holiday, Robert Wyatt, Louis Armstrong) but if we're strictly talking rock, it's gotta be Paul. His range is pretty breathtaking- like Elvis, he seems equally adept at rave ups (famously one of the best screamers in rock- see "Helier Skelter" and those early Little Richard-inspired hits) and ballads. And though in more recent years, Colin Blunstone has become my #1 singing influence, in the past it was always Paul who I modeled myself after.
As I've come to appreciate The Stones more in general with age (recently realizing that they've gradually become one of my favorite groups), I've also come to really appreciate Mick's singing. I always liked him but it still sorta took awhile to get used to how affected and OTT he often is- think that's probably pretty common. But he's really a master at selling the drama of those songs- I kinda think of him as more of an actor than a singer actually (singing's generally at least half acting anyway).
But I can completely understand your wife's opinion and there admittedly are moments when he's distractingly goofy like when he really goes for southern accent on some of their country numbers or when he over-emotes on ballads. And sometimes he just plain can't hit the note- like that low one in the verses of "Ruby Tuesday" (which is admittedly a bit tricky, kinda like the low note at the beginning of "Happiness Is A Warm Gun").