The zoo and Rouge Park do not and never will generate much transit ridership. There is not much traffic in this area and most of the people going here will probably drive anyway.
Most of the ridership on the Sheppard LRT will be between Don Mills and Kennedy, where I expect the LRT will be packed in rush hour due to too little capacity. There have been a whole bunch of condo towers built along Sheppard (including at Allen, Bathurst, Yonge, Bayview, Bessarion, Leslie, Don Mills, Victoria Park and Kennedy) and around Scarborough Centre in the last ten years. There have been a few condo towers at Sheppard & Markham Rd as well. Also the traffic on the 401 tend to be worst west of the 404, bad in rush hour between 404 and somewhere between Kennedy and Morningside, and uncongested between Morningside and somewhere just west of Brock Road where the collector-express lanes end. On the other hand the east end of the LRT will be underused as there is not much there other than the zoo, Rouge Park and some big box stores and housing subdivisons.