if you look again at the close-up of the sectional plan of Covent Garden, you can just about make out the figure of Tragedy( or is it Comedy?) to which Sheriden refers in those last four lines of the Second prologue.

He had already had Mrs Bulkey point to the figure of Comedy( opposite) in Line 7:

You can just make out the figure- it is to the right hand side of the stage as the audience woudl view it, just above the word "Proscenium"

These reconstructions were created by Richard Leachcroft for his bookThe Development of the English Playhouse

IMHO, here Sheriden is arguing for comedy trumphing over tragedy, which was often seen to be the more "respectable " form of theatre? That serious points can be made in the from of a comedy too?