The NKJV footnotes the opening phrase with a rendering that ties this verse to the famous poem at the end of the book.
Quick answer
A marriage shows. The verse says a good one lifts a man visibly and a bad one degrades him from the inside, and the images are chosen to be opposites.
Read both lines
The NKJV footnotes "excellent wife" as literally a wife of valor, and that is the same phrase the book uses at 31:10, where the NKJV glosses it "a wife of valor, in the sense of all forms of excellence." So the woman being described here is the acrostic's woman: capable, strong, enterprising. The two images then oppose each other precisely. A crown is worn on the outside, public and honoring; rottenness in the bones is interior, invisible, and structural. The verse is not rating wives. It is observing that the closest relationship in a life is not a private matter, and that its condition works outward or inward accordingly.
What it does not mean
The second line is harsh and has been quoted at women, so its limits are worth naming. It describes a specific dynamic, a spouse whose conduct actively brings disgrace on the household, and it is not a verse for grading an ordinary marriage or an ordinary wife. Note also that the parallel runs both ways in this book, which says elsewhere that a man's folly ruins his own house (14:1 speaks of a house built and torn down, and 11:29 of a man who troubles his own household and inherits wind). Neither line of this verse licenses anyone to appraise a spouse, and the first line, not the second, is the one the book develops into twenty-two verses of praise.
Related: Proverbs 31:10–31 · Proverbs 11:29 · Proverbs 18:22
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.