It's a failure.20% is not a huge number, thats still around 12m unit being produced after the "slash", and thats only INITIAL orders.
The iPhone Mini's orders were "slashed" similarly on release.
I'd guess during its lifetime they'll sell upward of 30m SE3's. Thats hardly a failure.
Heck, the SE3 is still not even available in some of the markets where its probably going to do extremely well.

Apple is already slashing iPhone SE 3 production due to weak demand
Apple is reportedly making big cuts to iPhone SE 3 production less than two weeks after the device made its official debut. Two separate sources claim
www.cultofmac.com