The meaning of "buenas" in "buenas tardes" or "buenas noches" is rather formulaic (Good afternoon, Good evening).
When the weather is not good, or there is some other undesired condition, the person may ironically use "aunque de buenas tienen poco" after the greeting to mean that the afternoon or evening in question is not actually so good. The person does wish the interlocutor a good afternoon or evening but at the same time admits that the situation is not agreeable.
The addition works like some kind of play on words, by putting together the first formulaic "buenas" with the second, semantically richer "buenas". I've also heard:
- Buenas tardes, si es que se puede decir "buenas" (Good afternoon, that is, if we can actually call it "good")
We can also use the masculine (singular or plural) for "good morning":
Buen día, si es que se lo puede llamar "bueno".
Buenos días, si es que se les puede decir "buenos".