Some lounges do have dress codes, for me most recently the RC Hong Kong which explicitly says no robes in the lounge. Other hotels say that the robe is to be used only in your room--for example, in Japan in genereal and previously in the Peninsula Hong Kong, which now allows the bathrobe when going to and from the pool/spa.

To me, the robe is OK for trips to/from pool/health club areas that don't cross the main lobby, etc. It also, with closed slippers or shoes, might be OK when the lounge first opens to grab some coffee and immediately return to the room, as opposed to staying to consume stuff in the lounge. (I'm thinking of the time when Hyatt Gold Passport floors had coffee and muffins on tables by the elevators and it seemed OK to sneak there to grab something to take back to one's room.)

Personally, I'm more upset by bare feet in lounges at any time as well as sweaty wife beaters or very short shorts sitting on upholstered furniture.