No that's "muddy"
Spongey is an amp term first and it means when you hit a note really hard,the amp gives up a bit of it's tightness and gives you a softer feel with some compression and a blossoming of the note again after the initial attack of the note....Another amp term for this is "sag".....
When guys describe A2 magnets,we or they,use the term "sweet"....Sweet has everything to do with the mids,lows,and how the pickup reacts under string vibration...
Hears another way to look at "Spongey"
An amp with a SS Rectifier stays tighter and has a tone that "Honks"
That same amp with a tube rectifier would possibly have more "sag" and not be as tight sounding...It's tone would be more like a "hunk" and not a "honk".....
It's hard to describe because it's all a feel thing and it's hard to put into words really....