General synonyms:
* Depressing
* Bleak
* Gloomy
* Cheerless
* Dreary
* Melancholy
* Grim
* Miserable
* Desolate
* Forlorn
* Wretched
* Sombre
More specific synonyms:
* For something uninspiring: Mediocre, lackluster, dull, uninspiring
* For something that is failing or unsuccessful: Hopeless, dire, bleak, desperate
* For something that is physically unpleasant: Dark, gloomy, shadowy, cold, damp
The best synonym for "dismal" will depend on the context of your writing. Consider what specific aspect of "dismal" you want to emphasize when choosing your synonym.