We've not done the ABD cruises, but we have done a few trips with ABD's provider, AMA Waterways. The vibe is definitely 'nice casual'. While we have seen people on these cruises come to dinner in blue jeans, the vast majority are in the 'slacks and a nice shirt' category. Slip-ons and walking shoes seem to be the go-to for footwear. Truth be told, there were some days when we'd arrive back on board after a long walk or bike ride and 'dressing for dinner' meant 'find something clean'. Remember, this is YOUR vacation, so do what you think is comfortable.
P.S. We used the laundry service on the ship and I recall the cost was reasonable enough.