Judicial activism is controversial because it is not clear if the practice is democratic.
Some people argue that judicial activism is undemocratic. They argue that judicial activism results in unelected judges overturning the will of the people. They argue that judges should not overturn laws created by the elected branches unless the laws are clearly unconstitutional.
Others, however, argue that judicial activism helps to protect our democracy. They say that judges need to actively strike down laws that are not constitutional even if they are supported by large majorities. They say that democracy depends on having an independent judiciary that will stand up for the rights of people who are likely to have their rights infringed upon by majorities.