If Kpop, is idol group-dancing songs, then only BTS has mainstream acceptance, by popularity and longevity in Korea.
Gaon data - year end digital longevity for the decade can be used to determine that.
2010-20, only IU TTN and BTS SD has been on the top 100 for 4 years
In case of 3 years, again only BTS is the idol-dance group, otherwise it is IU, Paul Kim, Busker Busker
In case of 2 years, finally the Kpop GGs.BGs are present - Twice, BP, Gfriend etc. But again the list is dominated by Soloists - Ailee, Gummy, Park Hyoshin and trottists, foreign singers.
But the definition of K-pop is fuzzy. It includes IU, but not Paul Kim.
So more than looking at whether Kpop is mainstream popular or not, it is specific genres, specific acts, specific songs.
Among idol-dance-groups, only BTS is consistently popular and has longevity, across demography.
Otherwise Kpop is limited in its reach, mostly to teenagers and 20ers.