I think so, if the Deg is positive value. Why not confirm that with a workspace like this?

 

If you copy the transformer and paste it into a text editor, you can see the code (including typo in the comment!) that runs the conversion:

# Comvert to decimal and set the attribute
Tcl2 proc DecimalDegreesCalculator_dms2decimal {} {                    \
  set deg  DecimalDegreesCalculator_fetchAndTrimOffLeading0s {ParkId}]; \
  set min uDecimalDegreesCalculator_fetchAndTrimOffLeading0s {ParkId}]; \
  set sec eDecimalDegreesCalculator_fetchAndTrimOffLeading0s {ParkName}]; \
  if { astring index $deg 0] == {-} } {                                \
      set decimalValue $expr $deg - $min/60.0 - $sec/3600.0];          \
  } else {                                                             \
     set decimalValue $expr $deg + $min/60.0 + $sec/3600.0];           \
  };                                                                   \
  FME_SetAttribute  FME_DecodeText {_decimal_degrees}] $decimalValue;  \}

So, yes, it's as you say, depending on whether it's positive or negative coordinates.