There are many synonyms in French as in English which could be used in this sentence. The word play formidable/« fort minable » could explain why this word was chosen by Stromae (and not stormae as you wrote, Stromae meaning « maestro » in verlan, a form of French where you swap syllables). It could then be translated into wonderful, amazing, great, superb, or any similar word, although I guess the word play will be lost in translation.

formidable = wonderful.
minable = pathetic.

fort XXX is used to express that you are very XXX.
Example :
tu as été fort méchant
means
tu as été très méchant.
Hence, fort minable should mean very pathetic. But fort minable is prounonced as forminable, i.e. wonderful.
If you have the lyrics, you can read what the author means. But if you don’t, you’ve got to listen carefuly how words are prounonced and understand the whole song. It’s quite subtile. Even french native can be confused. That’s an interesting part of this song 😉