Q. Why do Chassidim wear black and look untidy?
A. Looking untidy does not go with the territory. The rule in halachah is that a talmid chacham must be neat and clean without a speck on his clothes. Whatever colour a person’s clothes are, an orthodox Jew must not bring the Torah into disrepute by looking dishevelled. Some Chassidim are scrupulous with their appearance; others are not.
But the same applies to non-Chassidim too. Even the long black coats which are often viewed as a Chassidic practice, with or without a shtreimel, are not limited to Chassidim, nor are they intrinsically Jewish. The long coats are probably part of a dress code in the colder parts of Europe; the black colour may derive from the black clothes imposed by the Caliph Omar in the 7th century and by Pope Innocent III in 1215.
The problem is not whether one wears a particular colour but whether one is scruffy and unkempt. The orthodox may sometimes need reminding of this rule, but so do the non-orthodox. The latter also need to be reminded of the rules against immodest dress, especially amongst women.