I actually really love this concept, because of the kind of creativity it encourages. It's basically a blank palette to paint on, and though I understand that there's not really a "direction", I think it really gets the brain juices flowing :)
I think we would have to not simply theorize about "mediocrity", but lay out and define what actually is "mediocre." Like, how high of a BST is too high, and how high can you put a single stat before it stops being "mediocre"? (Maybe average out stats in OU, and put all stats of this 'mon below that threshold?) What typing or type combination is "mediocre"? What constitutes a "mediocre" movepool, and what moves are "mediocre"? What abilities are "mediocre"?
If we're looking at the current average for OU, an average mon has a BTS of about 570 tending towards physical aggression with 3 regular weaknesses, 4 regular resistances, a single immunity, and potentially either a quad weakness or a quad resistance. Note that these are unweighted averages not taking into account frequently held items (though it does take into account the Huge Power on Azumarill and abilities that add immunities). Going off this, mediocre defensive typing would include:
Monotyped: Bug, Dark, Dragon, Fighting, Flying, Grass, Ground, Ice, Psychic, and Rock
Dual-typed: Bug/Dark, Bug/Dragon, Bug/Fairy, Bug/Fighting, Bug/Ground, Bug/Ice, Bug/Normal, Bug/Psychic, Bug/Rock, Dark/Fighting, Dark/Ice, Dragon/Flying, Dragon/Ground, Dragon/Ice, Dragon/Rock, Electric/Fairy, Electric/Grass, Electric/Ground, Electric/Ice, Electric/Rock, Fairy/Fighting, Fairy/Ice, Fairy/Poison, Fairy/Psychic, Fighting/Flying, Fighting/Ground, Fighting/Ice, Fighting/Normal, Fighting/Psychic, Fire/Flying, Fire/Grass, Fire/Ice, Flying/Ice, Flying/Psychic, Grass/Ground, Grass/Ice, Grass/Normal, Grass/Poison, Grass/Rock, Grass/Water, Ground/Ice, Ground/Poison, Ground/Psychic, Ice/Normal, Ice/Psychic, Ice/Water, Normal/Rock, Normal/Water, Poison/Psychic, Rock/Water
This is just as defensive typing mind you. Now let's look at offensive typing. Dark, Dragon, Fairy, Fighting, Ground, and Water all tend to be on the strong end of offensive typing. Specific combinations of types such as Electric/Ice should also strike you as a tad excessively good. Normal/Rock and Grass/Ice are also on the list, which should strike you as far below mediocre. Eliminating those, our list becomes:
Monotyped: Bug, Flying, Grass, Ice, Psychic, and Rock
Dual-typed: Bug/Ice, Bug/Normal, Bug/Psychic, Bug/Rock, Electric/Grass, Electric/Rock, Fire/Flying, Fire/Grass, Flying/Ice, Flying/Psychic, Grass/Normal, Grass/Poison, Grass/Rock, Fire/Ice, Ice/Normal, Ice/Psychic, Poison/Psychic
So this should be about the list of types that are okay but not incredible.