I have a Mini 6 with the jelly scroll issue. I didn't notice it until someone posted about it in macrumors forums. I can replicate it now, easily.
But it doesn't bother me. Now, the PRINCIPLE bothers me, that I would spend $650 on a device that has this issue. But since becoming aware of it, I have spent many hours testing all the apps I'd normally use and all the things I'd normally do on an iPad Mini, and none of them are impacted by this issue. So I won't be returning my Mini 6.
Lots of people are complaining about Apple for using substandard displays in such an expensive device. But if you don't own a Mini 6, you don't know how much if at all the issue would bother you.
The people who preordered the Mini 6 and already have it in their hands, and have put it through its paces, these are the ones who can best say how bad the problem is.
And it's clear from jelly scrolling discussion in macrumors forums that there's is a huge range of responses among owners, from people who find the effect intolerable to those like me who can totally live with it, and everything in between.
This wide range of responses suggests to me that the effect is worse on some iPad Mini 6 units than others. (It's also surely true that people's eyesight and sensitivity varies, and the way they use the device and the apps they use varies.)
So if you had wanted to buy a Mini 6, why not buy it and see if the effect is bad on the unit you get. If it is, then definitely return it. But it might not be. You might be happy with the unit you get. There are lots of Mini 6 owners who don't find the effect bothersome at all, and among the professional reviews posted last week, few if any even noticed the issue until it was later pointed out, Friday or Saturday.