Mr J wrote, "...since faves usually attract the mug punter, doesn't it make sense that it's more likely value is on the other side?"

In horse racing (throughout the world except Hong Kong) the mugs back anything but the Favourite. To illustrate consider the following numbers:

Sat Metro races - last three years, NSW TAB divs

SP under 11.0 SP over 11.1

5477 races 5448 races
4370 winners 1107 winners
SR 79.8% SR 20.3%
LoT 15.0% LoT 26.7%

Just backing the Favourite reduces the LoT another couple of % points.

Which section of the market are the mugs slothing around in? Which section of the market do you want to concentrate your efforts on?

Sports punting can be different. You are familiar with the Football-Data site that proved backing short priced Soccer teams (<1.3) was profitable irrespective of form, injuries, condidtions, H/A etc etc etc so certain sections of Sports betting also exhibit a fav-longshot bias.