There are malls that are well serviced by transit; Yorkdale and Fairview both have subway stations at their door; STC has the SRT (for now); I think a parking tax, within Toronto, where transit is more comprehensive probably does make sense. It doesn't need to be high to be useful.
Take Fairview, I dont' remember what the current number of spaces is; but it used to about 4,200.
At $1 per day, per space (which with turnover is well less than $1 per parker/customer) you generate $4,200 per day which is about $1,500,000 per year.
That's for just one mall property.
For a (slightly out of date) analysis of potential issues with and revenue that might be derived from a Parking Tax, see this KPMG report for the City of Toronto.
Parking Tax Discussion and precedents begin on page 48.
From said report:
View attachment 310289
I favour this tax, along with highway tolls.
I don't oppose a downtown congestion charge in theory; though I think its more cumbersome to implement and does nothing for suburban congestion.