There are a few rhythm guitar players (including Delbert Williams, a very respected California musician), who do use a thumb pick, in the style of some of the earliest bluegrass guitar players, such as Lester Flatt and Carter Stanley. But these exceptions are rare.
I would say a flat pick is not *necessary* for bluegrass, but the standard way the guitar is played in bluegrass does *not* call for fingerpicking at all. Instead, it is typically a strong and clear bass note followed by a quick and clean (not noisy) brushed strum on mostly strings 1, 2, and 3. The idea is to have the low note, and punctuate time, not to “fill” the midrange sound of the ensemble.
Generally, people make the bluegrass guitar sound with a flat pick, but if the grip, etc. are awkward at first, you can try for the same sound with a thumbpick and maybe a single finger pick for the quick strum (not individual notes). Sorry to sound dogmatic about that, but I feel responsible for helping people to understand and learn “the bluegrass way” from me, which they then can use according to their own judgement.