The question is not so much if comedy is valued over tragedy in Samuel Johnson's "Preface to Shakespeare"—Johnson makes no value judgement between and tragedy—but in what ways did Johnson believe that Shakespeare was a better writer of comedic scenes than of tragic scenes.
Johnson makes no distinction between Shakespeare's comedies and tragedies.
Shakespeare's plays are not in the rigorous and critical sense either tragedies or comedies, but compositions of a distinct kind; exhibiting the real state of sublunary [down to earth] nature, which partakes of good and evil, joy and sorrow, mingled with endless variety of proportion and innumerable modes of combination . . .
Johnson notes that in the works of the ancient Greek and Roman playwrights there were only two types of plays—comedies and tragedies—and each type of play was clearly distinct from the other, "according to the laws which custom had prescribed . . . and considered as so little allied, that I do not recollect among the Greeks or Romans a single writer who attempted both."
According to Johnson, Shakespeare's plays were different.
Shakespeare has united the powers of exciting laughter and sorrow not only in one mind, but in one composition. Almost all his plays are divided between serious and ludicrous characters, and, in the successive evolutions of the design, sometimes produce seriousness and sorrow, and sometimes levity and laughter.
Johnson believes that Shakespeare is a natural, intuitive, instinctual writer of comedy and comic scenes, and that, although Shakespeare is very skillful at writing tragedy, he clearly labors over the tragic elements of his plays.
In tragedy he often writes with great appearance of toil and study, what is written at last with little felicity; but in his comick scenes, he seems to produce without labour, what no labour can improve. In tragedy he is always struggling after some occasion to be comick, but in comedy he seems to repose, or to luxuriate, as in a mode of thinking congenial to his nature. In his tragick scenes there is always something wanting, but his comedy often surpasses expectation or desire. His comedy pleases by the thoughts and the language, and his tragedy for the greater part by incident and action. His tragedy seems to be skill, his comedy to be instinct.
Johnson contends that Shakespeare's comic characters are more natural, more true-to-life than his tragic characters, that they speak more like real people, and that they "hold a mirror up to nature" to a much greater (and more pleasing) degree than his tragic characters.
Nevertheless, Johnson believes that Shakespeare was successful in affecting an audience in all of his plays, whether a comedy, tragedy, or history.
Through all these denominations of the drama, Shakespeare's mode of composition is the same; an interchange of seriousness and merriment, by which the mind is softened at one time, and exhilarated at another. But whatever be his purpose, whether to gladden or depress, or to conduct the story, without vehemence or emotion, through tracts of easy and familiar dialogue, he never fails to attain his purpose; as he commands us, we laugh or mourn, or sit silent with quiet expectation, in tranquility without indifference.