At first, Angelo's name may seem ironic given that he's no "angel" in this play, but, if we think of an "angel" as a non-human or other-worldly figure, then the name might begin to make more sense. At various moments in the play, Angelo is characterized as being, well, inhuman. He claims to have never had any kind of sexual appetite (until Isabella came along) or any other kind of natural desire and he certainly acts like he's super-human. This is probably why Lucio spreads a nasty rumor about him:
Some report a sea-maid spawned him; some, that he
was begot between two stock-fishes. But it is
certain that when he makes water his urine is
congealed ice; that I know to be true: and he is a
motion generative; that's infallible. (3.2.14)
Angelo's name is also associated with a type of coin (an "angel"), which you can read more about by going to "Symbolism, Imagery, Allegory."
Mistress Overdone's name, as you've probably guessed, is fitting, given that she's devoted so much of her life (and her health) to her trade. As Pompey tells us, Mistress Overdone "worn [her] eyes almost out in the service" (1.2.8). In other words, Overdone's vision is impaired because she's in the late stages of syphilis. (Anyone familiar with Mistress Quickly from Henry IV Parts 1 and 2 knows that Shakespeare likes these kinds of bawdy names.)
Characters' sex lives are a big deal in Measure for Measure. On one end of the spectrum, Shakespeare portrays characters (like Mistress Overdone) who are guilty of fornication (a crime in Vienna). Overdone's participation in the sex industry makes her a figure to be mocked (or potentially thrown in jail). On the other end of the spectrum, we find characters who abstain from sex altogether, like Isabella, who seeks refuge at a convent. (Angelo also seems abstinent until he's overcome by his lust for Isabella.) The interesting thing is that, according to the play, Isabella's desire for permanent virginity seems just as extreme as Mistress Overdone's hyper-sexuality.