This phrase, "hooked in the side," means that she was poked or probably stabbed in the side by the horn of the cow she was milking. She adds, "Cows are soothing and slow and don't bother you, unless you try to milk them the wrong way." This is pretty much the end of this anecdote. But this could be symbolic of Mrs. Johnson's tough determination. She says she "used to love to milk" until the accident. But she doesn't say she has stopped. So, even though she got hurt (bad enough to make her dislike the act), she still does the work.
We might also use the notion of being "hooked in the side" metaphorically. She and Maggie have a different worldview than Dee does. So, when Dee comes to visit, there is some conflict. Dee has consistently tried to modernize her mother and sister, but they inevitably prefer their rural, country life. The times when Dee is condescending could be inferred as Mrs. Johnson being figuratively "hooked" by Dee. For example, when Dee leaves, she dismissively says, "You just don't understand." Or, she might feel "hooked" when Hakim-a-barber refuses to eat her food. Since "hooked in the side" means being injured, it could be used in comparison for the times when Mrs. Johnson is insulted or emotionally injured. But, even though she is insulted by Dee, she still wants her to visit because she still loves her daughter. Likewise, even though she is injured, she still continues to milk the cow. In both cases, she has been injured, but still tries to get something "useful" out of the interaction: milk or a connection with Dee.