People ask about Patricia Heaton's hair in The Middle because Frankie Heck's look is intentionally unglamorous, practical, and consistent. That does not automatically mean a wig was used.
Why sitcom hair looks suspicious
TV hair departments maintain continuity across episodes shot out of order. Even "messy mom hair" is designed and repeated.
What is safe to say
Unless a direct stylist or actor source confirms a wig, the best answer is that the look was a controlled character hairstyle. It may have involved professional styling, but a full wig claim should be treated carefully.
Bottom line
There is not enough strong public evidence to state that Patricia Heaton wore a wig in The Middle. The hair is better understood as character styling.