In the play "On the Face of It", why did Derry's mother not allow Derry to go back to Mr. Lamb? What had she heard and been warned about Mr. Lamb from others?
The story doesn't say.
In all the conversation between Derry and Mr Lamb, they talk about people's negative reactions to the boy with the burned face, but nothing about people's negative reactions to the old man with the tin leg. Perhaps this reflects the two characters' different outlooks on life. Mr Lamb is relentlessly positive and optimistic: he shows Derry that he can be as successful as anyone else despite his burned face; he loves all of his plants in the same way, whether they're "weeds" or flowers and fruits; he speaks positively about the people he knows too:
MR LAMB: Friends everywhere. People come in.... everybody knows me. The gate’s always open. They come and sit here. And in front of the fire in winter. Kids come for the apples and pears. And for toffee. I make toffee with honey. Anybody comes. So have you.
It's possible that his optimistic outlook conceals some more negative things as well. Clearly, not everyone likes or trusts him. But he's not the kind of person to care about the negativity of his neighbours. He's fine with everyone, whether they like him or not. He's a person to see the good in everyone, and not to care about the bad.
Since his words are about the only insight we get into his own life, we don't know about the things he doesn't care enough to talk about, like people's negativity towards him. The only hint we get is from the words of Derry's mother:
MOTHER: You think I don’t know about him, you think I haven’t heard things?
DERRY: You shouldn’t believe all you hear.
MOTHER: Been told. Warned. We’ve not lived here three months, but I know what there is to know and you’re not to go back there.
My interpretation is that closed-minded people don't appreciate open-minded ones. People who are open-minded, who think differently from others rather than accepting how society tells them to think, are often spurned or even hated by others around them. Those who don't or can't think for themselves, who live in echo chambers or accept standard views unquestioningly, tend to resent those who think outside the box - envying them, or not understanding them, or not liking to have their own assumptions challenged.
Derry is in a situation to appreciate Mr Lamb. He's young enough for his mind to be still fairly supple, not fixed in stone by the norms of adulthood. The unthinking assumptions of people around him have generally had a negative effect on him - people who judge him by his face, strangers who say he will never be loved, parents who think he won't be able to manage in the world when they're gone - so challenging those assumptions can only be a positive thing for him.
But others who know Mr Lamb may not have been so appreciative of his wisdom. Maybe they kept the same attitude as Derry had at first: angrily refuting the old man's unconvential statements. I could imagine a closed-minded person thinking "everyone knows weeds aren't worth as much as flowers, what is this stupid old man talking about?" or "of course people aren't all equally valuable, why did I work so hard to become an important person in society then?" Challenging people's assumptions, especially those unspoken but unthinkingly accepted, always gets them edgy.
Maybe other children came to Mr Lamb's house and enjoyed learning from him and his fresh outlook on the world, but then came back home to their parents with some of those "difficult" views. The only thing more uncomfortable than meeting a stranger challenging your worldview with his open-mindedness would be having that same challenging open-mindedness in your own home from your own children. Families have been estranged over less. It could have been some such parents who "warned" Derry's mother about Mr Lamb and his dangerous influences.
Another possibility is that there really is something bad about Mr Lamb that we never have the chance to discover because he dies before Derry can get to know him better. This tantalising conclusion to the story leaves so many questions unanswered and unresolved. Like Derry, we're left feeling that here was a fascinating personality that we could've enjoyed getting to know better - but maybe more time with him would've revealed some unexpected negative aspects. This interpretation could be supported by the following line Mr Lamb utters to himself just after Derry leaves promising to come back:
MR LAMB: There my dears [his bees]. That’s you seen to. Ah....you know. We all know. I’ll come back. They never do, though. Not them. Never do come back.
Does this suggest Mr Lamb was lying when he said "Anybody comes" to his home, to sit with him and share his food? It could be interpreted as meaning he has something to hide.
It's still consistent with my interpretation, though. Even if nobody comes back to him, it doesn't mean he's not worth coming back to. It could be the same attitude we see from Derry's mother, rather than any real problem with him, that keeps them from coming back. Some of them dislike his open-mindedness and prefer not to be exposed to it; others, such as children, may be prevented from returning by their parents.