That's not her actual voice in Mike and Molly
It can't be.
When she has sung on the show it is probably her singing. Sometimes her actual voice shines through, like in one scene where she and Melissa McCarthy were sitting in caskets and she said "Good evening, Miss Flynn," in a much deeper and resonant feminine tone than her nasally speaking voice.
What kind of replies am I going to get? Duh, or You're wrong?