It is OP in circumstance so not every time is it OP but turning a 4/4 cockatrice and turning a 10/10 dragon are two very different things in the first case its only a 3/3 reduction due to the fact that it is unupped but sense it doesn't remove the creature from the space it can lower your over all effectiveness, the second case turns a creature that cost 12
into a 1/1 creature for 2
that is a massive reduction its more than an CC can do even reveres time lets you get it back but this doesn't allow that to happen yes antimatter lowers total damage more but it is less effective due to the fact it is more expensive at a minimum of 4
when used as an ability from the nymph
To me it just seems like punishment for using expensive creatures just to make them next to useless