This is because he might tend to say troubling things that might give people the wrong idea about what goes on at the Pattermanse. The first such thing would have to be "Mike's mom doesn't like having him or Lizzie inside very much an'she gets real angry if he comes inside before it gets dark out. One time, Mommy an'Daddy were talkin'when I wasn't s'pposed t'hear'em an'he said that she didn't want kids, she wanted little dummies she could put in a glass box when she was done playin'with them." The implication that Elly is a fragile idiot who can't coexist with children who inspire anger and hatred within her heart is not consistent with her self-image so is just gibberish Lynn can't hear.
The second disturbing thing is probably "Mike Patterson might think that he's some great big important guy who can't be seen with us or his sister but most of us kids think he's this real big loser who can be talked into doin'stupid stuff 'cause he's too dumb to come in outta the rain." Now, Lynn might secretly believe this and tends to imply that Mike's need to get in good with the guys makes his life worse but to actively say so out loud is cruel and wrong.
So is "We like having Mike as a baby-sitter because it's funny when he goes all to pieces when we get rowdy. Dad says it's like havin'a small version of his mom over because she's fragile too. Funny. I thought it was just stuff that was fragile. I didn't know people were fragile but Dad says Mrs Patterson is. It sure is bad to be Lizzie, though. She's told not to stand up for herself because her parents want a kewpie doll instead of a kid (Mom AND Dad say that) and she's 'bout the only person Mike can push around an'get away with it because his dad 'cowers behind his paper' and his mom's got her head lost in some place called the Twilight Zone and they just let him do stuff." You can readily see that he has nothing to say to Lynn when he says something like that. The very idea of calling John and Elly absentee parents who let chaos happen because they're asleep at the switch is just gibberish Lynn can't take seriously