The short answer is, they don't-- they reveal PART of his opinion.
To expand on that a little bit, Theseus definitely thinks the players are foolish. He mocks them at various times during their presentation, and that must be taken into account. However, these lines express a kind of warmth towards the players, and a self-awareness: since all acting involves playing at being something you're not (shadows), all of it depends on the audience's attitude (imagination mending it). That would apply to him too, and would remind the audience of their need to be kind, but as far as the players, the lines show Theseus willing to forgive them their silliness due to their good intent.