The phrase "a mild success" is correct and usable in written English.
It can be used to describe an achievement that is positive but not overwhelmingly so, indicating a moderate level of success.
Example: "The event was a mild success, attracting a decent number of attendees but falling short of our expectations."
Alternatives: "a modest success" or "a slight success."
While Minnow was a mild success, it was Tom's Midnight Garden that really took off.
I had made — despite it all — a mild success of myself in life.
One of his campaigns, Superman Is an Immigrant, had been a mild success.
It scored a mild success, and the puzzle was made a regular feature of the paper.
Perhaps a result of what seems to have been only a mild success with her own companies, Barriscale left the screen.
One of the songs Porter wrote for the show was "I'm in Love Again," which later became a hit, but the "Follies" was only a mild success.
A run of mild success has startled many Knicks fans into a state of misguided euphoria, but it's worth remembering a time in the early nineteen-seventies when the franchise really did rule the basketball world, fielding two championship teams composed of fascinating players who — thanks to their inspired performances and geographic proximity to the media hive — were elevated to folk-hero heights.
He's been playing at the pass-the-hats in the Village, trying to flog a solo career, having had mild success as part of a duo.
This new expenditure in accountants, lawyers and IT infrastructure is a significant penalty for mild success.
McCarthy spent her remaining time in Congress trying to revive it and other gun laws before retiring in 2015, with only mild success and a growing number of mass shootings.
"Does that depend on a president?" Then, after a little more back-and-forth, the candidate tied it all in with the feel-good mantra that has been his unlikely ticket to mild success in a GOP race in which the loudest, angriest voices have otherwise prevailed.
