Many affiliate browser extensions do indeed do this, as an extra revenue stream. In fact, I'd recommend never installing a coupon browser extension. But replacing one number with another does not meet the above definition of spyware.
"Programs designed to monitor users' web browsing, display unsolicited advertisements, *or redirect affiliate marketing revenues* are called spyware."
> Spyware is a form of malware that hides on your device, monitors your activity, and steals sensitive information like bank details and passwords [0]
> Spyware is loosely defined as malicious software designed to enter your computer device, gather data about you, and forward it to a third-party without your consent. [1]
> Spyware is malicious software that secretly monitors your activity and collects sensitive information, like passwords, location data, or browsing habits, without your consent. [2][3]
0: https://www.malwarebytes.com/spyware
1: https://usa.kaspersky.com/resource-center/threats/spyware
I already thought Honey was scummy so I never used it in the first place, but I honestly don't get the particular outrage over these specific practices. You're already using the extension to effectively scam online stores, by using coupons the company gave to somebody else, not you. I see it as barely more ethical than doing that old trick of generating your own manufacturer coupons. Probably it's a lot more legal, but ethically it's in the same ballpark.
I don't know what the ratio is, but I do know it doesn't matter in this context, it's still malware.