I don't know enough about the language of the era to apply this to "awful," but in Northanger Abbey the word "nice" appears to be undergoing some transition in its usage, and I believe elsewhere in this GR (or recently on the P&P thread) someone noted that "stupid" as Bingley uses it in Chapter 3 does not mean the same as it does today.

Perhaps "awful" was transitioning from "awe-ful" to "dreadful," and may not have been as severe a criticism as we understand it today. If it was a word in transition, the newer meaning might not be noted in a contemporary dictionary. Would appreciate further commentary, if anyone knows more about this.

If "stupid" did have similar meaning then as now, Bingley is not above (jokingly) insulting his friends!