frump/frumpy
because one of the two definitions of "frump" is an unattractive girl/woman and because the terms are commonly reserved for women, you may need alternatives: slob, sloven, unkempt person, stick-in-the-mud; dowdy, slovenly, unkempt, tacky, drab, old-fashioned, unfashionable, out-of-date, staid.
