There are several *huge* and unverified (unverifiable) leaps here:

1. The idea that all good composers are smokers - unverified
2. The idea that no non-smokers have produced great art - unverified
3. The idea that non-smokers produce bad music - unverified
4. A causal connection between smoking and 'good' art - unverified
5. A comparison of smoking rates among artists and the population at large to show significantly higher smoking rates among artists - unverified

Just because someone makes a claim - "it is hard to name a single worthwhile piece of music written by a non-smoker" - does not mean it is true.

The claim is as silly as that only poor people produce great art (poverty gives you soul, apparently), or that men are better drivers than women (the statistics give the lie to that).

Did you know that 58.7% of all statistics are made up? ;-)