The Transformation Profile is “one way”, used with MAP it will take a state channel update from a binding and transform it, before updating the Item state.
It doesn’t affect commands in any way.
My question about your command still stands, and the fix advice remains.
From your description though, it seems you’re expecting the MQTT binding to act on the string Item state, and there is no command involved at all? It just won’t. This binding wants commands, like most bindings.
You can work round that with a rule, but as you’ve discovered profiles, use that instead.
Link your string Item to your (edited) MQTT channel using a follow profile, this does the magic of converting Item state updates into channel commands.
You’ll need a different MAP for the MQTT channel to the Darksky channel though, text->numeric and numeric->text. Thinks; in fact you could both conversions in one xxx.map file for convenience.
This all seems rather roundabout; I think I would have used a Number Item to hold the raw ConditionID from Darkshy, with a MAP pattern for human readable display.
You may need to reboot after making all the changes you want, to untangle everything.