Do you think conan doyle has made sherlock holmes smart by dumbing down all the other character

No, Holmes is naturally clever and not at the expense of other characters. The cases are pretty tricky. For Holmes to succeed as Doyle's protagonist/hero he has to function a cut above everybody else. Watson, for example, has great powers of perception. Although he never achieves Holmes's stature, he does help Holmes solve the crimes. I don’t recall any characters as being inordinately stupid.