30 is hot,
20 is pleasing,
10 is not,
0 is freezing
As an old English major and newspaper editor—why abandon parallel structure on the fourth line? Rather than begin with an "and," just match the "at."
The poem already naturally clunks to a stop at the word "freezing," as it should, so you don't really need the additional signal to the reader provided by the "and."
I still like your poem better, but the "20 is cool" one had a 30-year head start.
The one I would PREFER, is the "... At 10 it's cold; And 0 it's freezing"; however, the one that is most easiest to remember is in fact the one "...10 is cool; 0 is ice"