"not the first negative reviews she has had lately."
She's always had negative reviews. They haven't stopped her becoming a global phenomenon, like her or not.
No she doen't have one of the greatest voices, but then most pop singers don't. The majority can hold a tune but sound a bit ropey when shorne of studio production. Madonna isn't a great live singer, but the fact it doesn't sound great - just okay and occasionally pretty good - underlines she doesn't mime, contrary to some claims.
She does do a big live production but I wish acts like her and Kylie M would occasionally do straightforward fronting-a-band gigs in smaller venues. I think KM did a few a couple of months back and Madonna is doing a small theatre show in Paris this coming week so who knows.