You need to find the correct character set where the degree symbol is defined. On the ZPL User Guide, the degree symbol is defined on the Zebra Code Page 1252, where the degree symbol is defined as B0 in HEX (or 176 in DEC). Below is the modified ZPL that prints the degree symbol. The character set is specified by ^CI27, use ^FH_ to use the HEX value _B0 for the degree symbol.

^XA
^LH0,0
^FO50,50
^CI27^A@R,20,20,TT0003M_^FO200,100^FR^FH_^FDStore refrigerated 2-8_B0C (36-46_B0F)^FS
^XZ