Mexican cannabis is grown outdoor without much care, while most commercially grown cannabis is grown in greenhouses or for small growers under artificial light (or even in hydro setups).
Most people today do not go to their dealer and ask for the kinds of cannabis they have. They just take whatever is there.
Not sure why you think most people don't ask their dealer what kinds of cannabis they have, but even if it's just one kind, I assure you they know what kind it is. The difference between Mexican Sativa and home-grown Indica is extremely obvious at a glance and the price differential is 2-5x.
The cannabis market doesn't match Akerlof's conditions. Due to the high rate of repeat business and relative ease of evaluating quality post-transaction, reputation functions as an effective public quality assurance mechanism.
Forced reliance on word-of-mouth advertising is very important as well in avoiding the market for lemons problem, but I don't think Akerlof considered that.
Most people today do not go to their dealer and ask for the kinds of cannabis they have. They just take whatever is there.