I'm quite surprised at how much I enjoy seeing those alerts and finding someone "liked" what I posted. I like changing my signature often and would love to see how many like it. Sorry, doesn't take much to entertain me.
I think you have to look at the practical uses of the like system and why people enjoy it so much. It's simple, you like a post, job done.
How would we like signatures? With the constant changing of signatures how would you know which ones were liked? and if a like structure was enforced where would it be displayed? I feel the the practicality of this suggestions needs more thought put into it rather than simply saying i want to like signatures. Why? where? how?