Quick answer
Some things do not belong together. The verse names two arrangements that look wrong because they put weight where it cannot be carried.
Read both lines
The word translated "fitting" is about propriety rather than law, so the complaint is aesthetic before it is moral: this is a picture out of order. A fool with luxury has resources and no judgment about them, which is a description of damage waiting to happen. The second line escalates from unsuitable to absurd. It is worth reading honestly alongside 17:2, where a wise servant does rule over a shameful son and shares the inheritance, because the book is not making a rule about social class in either place. There a servant rises by competence; here the arrangement is simply upside down. Proverbs is watching whether the person fits the weight, not whether he fits the category.
Related: Proverbs 17:2 · Proverbs 26:1 · Proverbs 30:21–22
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.