This is true of the few occasions when they actually deal with their opposite numbers, but in the more common instances where they attempt to infiltrate a group of eccentric enthusiasts who are going to use their eccentric enthusiasm to take over the country, Steed will often go in all Tim Nice-But-Dim. And yes, the heroic fop goes back at least to the Pimpernel, but there too it was always part of the trope that people assumed Percy couldn't possibly be the dashing Pimpernel, because he was such a wilting degenerate.
(As against eg Adam Adamant, who was just a straightforward hero with good shirts)