Why yes, I am an adult who reads children books.
I will be 80 and still reading picture books. It doesn't matter that books like this are targeted toward preteens or teenagers. I think it is sad if we get to a point in our life where we say "Well I'm XX now, can no longer enjoy that thing I used to enjoy..." It's fine if we naturally outgrow things of course, but don't force yourself to do so if you don't want to.
Judy Moody is a series I could have read when I was around the "right" age, not a preteen, but a teenager. I never read it because of pressure of trying to read things for my age or things that were above my reading level.
I was Judy's age once. It is fun to read about her silly adventures and put myself in her shoes. I never want to get to the point where I can't relate to younger (or older) characters.
Judy and her family are great. Kids of all ages, and kids at heart should get to know them.
*I read this twice. Once from the library and once provided by Netgally*