Pal I don’t like either one of them and I do consider them medicore talent at best and that comes not only from me but people who did play and played well.
You might have an elevated view of "mediocre." If the Marsalis brothers were on the bandstand next to Bix, or Diz, or Bird, or Satch, or Monk, they'd get cut. That would be a case of talent vs. genius. But "mediocre" is not a word I would apply to the worst soloist at the Metropolitan Opera.
Even in the most competitive Golden Age of jazz, the Marsalises might not have been star soloists, but they would have, at the very least, been second-chair in a top band or first chair in a smaller band. Reliable session cats. Like the vast majority of talented, highly skilled, professional musicians at any time in history. It isn't fair to label any cellist "mediocre" whose name doesn't end in Ma.
I don't listen to a whole lot of the Maraslis brothers, honestly. They're not very adventurous, and I tend to like my jazz out on the edge -- Monk, Mingus, Coltrane, Miles, and so on. Brubeck. I don't listen to that much poresent-day jazz, because too many "keepers of the flame" are as much historians as artists, and treat the music like a museum display. Frankly, why would I want to listen to someone trying to play like Satch when I can just listen to Satch?
Oh, and Kenny G wouldn't know jazz if it rolled up in a New Orleans second line and bit him on his talentless ass. That's not really relevant to what we're talking about, but it has to be said.