As cleverly alluded to by @Hugh in a comment above, closely related to “winning by default” is “winning by/in/with a walkover,” which according to Wikipedia is a term having its origins in the “Sport of Kings” to describe a race where the winning horse need only “walk over” the finish line to be officially declared the victor of a race “because there are no other [horses], or because the other [horses] have been disqualified or have forfeited.”
Quoting further from Wikipedia (emphasis added):
The word is used more generally by extension, particularly in
politics, for a contest in which the winner, although not the only
participant, has little or no competition.
For a phrase that I think goes well either alone or with “winning in/with/by a walkover” (and with “winning by default,” for that matter), and which doesn’t require extension of the literal meaning or modification of either of those terms (with “virtually,” for example) there is “winning by just (or simply) showing up”:
They won [in a walkover] by just showing up.
This is not the expression as it is used in this “Evolation Yoga Blog” in the sense of winning a moral victory or as part of a philosophical observation or pep talk (like Woody Allen’s “Eighty percent of success in life is just showing up”);
but rather as it’s used in a scenario as you describe, where the level of quality of the competitor(s) makes it possible for someone/some team to win (or not, in the case of the Penguins mentioned in this ‘Trib Live’ report) by just showing up.
Finally, again borrowing from terminology used in the “Sport of Kings, the notion of “running in/winning a maiden race” could describe your scenario, where a “maiden race,” as “confirmed” by Wikipedia is a race where none of the horses have ever won a race.
This LA Times article describing the 1987 Kentucky Derby describes well the [un]importance of “winning a maiden race” as follows in paragraph 12:
Winning a maiden race [at Turfway Park, the racing equivalent of the
old Three-I League,] is not enough to make an owner think he will some
day see his colt's name in gold on the clapboard walls of Churchill
Just in case you’re still looking for other options, the 1987 Kentucky Derby, judging from the article’s interesting description of it, greatly resembled your scenario, and the author used several colorful terms & expressions that you might find helpful, including:
“[it wasn’t so much won, as it was “inherited”] (winning by inheritance?);
“[winning] in/among a field of nobodies”;
“[winning] by surviving it/survival”;
“[winning] by being the most sober waterfront bar patron at closing time”;
“[winning] by being the best dancer in _____”; and
“[winning] by being the best skier in _____.”
(the last two being of an offensive nature, in my opinion, you’ll need to read the article to see how the article’s author filled in the blanks).