Is " the banning of performance enhancing drugs in baseball has decreased the number of home runs hit each year " a good thesis? If not explain why

It is not. Though the first part "the banning of performance enhancing drugs" is a good start, what follows seems to be an example rather than an argument to be explained. Remember that thesis statements are intended to express your opinion and give a general idea of the arguments that support it. These goals are not fulfilled, though it seems the author is against the banning, it is not explicitely said. And the arguments are not present. Also, the information is not logically presented.