What makes eclipse strong is that you use lots of cheap darkness (and death) creatures with relative low attack + vampire ability to get a strong offence with eclipse as well as good healing with buffed vampires. That said, rush-stile.
When mutation is used rushy, you won't get many mutants, so the effect is weaker than eclipse. Also, you won't get vampire that often, so that makes it weaker than eclipse as well. (And it gives only half the attack of eclipse, so again, weaker.) And one more thing, eclipse will most of the time buff ALL your creatures (in mono dark, and death/dark duos) while this card only buffs a part of your army (the mutants)
When used by stalling, this card won't be of much great use because, if you can live with a mutant stall long enough, you will win anyway. (Looking at scaredgirls rainbow)
With all the things that I counted just now, if it stacks, I think it's pretty balanced, and if not, it's UP. (But that is just my feeling)