I don't think it's too much of a stretch to claim that people riding cheap bikes in normal clothes with no helmet and little care for the finer points of roadcraft do exist as a broad category. So do overcautious middle-aged people on mid-range hybrids wearing cheap helmets and fluorescent jackets. So do arrogant MAMILs wearing full kit, expensive helmets riding carbon fibre twatmachines. So do helmetless bearded old roadies on steel tourers.
These stereotypes do exist and it is possible to broadly predict their behaviour based on appearance. But guess what? There are also exceptions and other cyclists who don't fall into these very broad stereotyped categories.
As long as you don't rely on it totally, the appearance of a cyclist, including the wearing or not of a helmet, can give you clues as to their likely or possible actions on the road. We do it all the time with drivers as well, teenagers in hot hatches or whatever are likely to drive differently compared with pensioners in crap saloons. It's part of roadcraft, predicting and mitigating the actions of others.
