The naming process often boils down to balancing a decision matrix of objective and subjective trade-offs between positives and negatives for a given scope, audience, market and requirements (legal, perceptual, cultural, linguistic) as well as constraints (time, money, creativity). These tend to be priced in different, 'currencies' and individual preferences that are challenging to reconcile.
As someone who has done quite a bit of pretty successful product and company naming, the choice "Mmhmm" has quite a few negative aspects which I consider to be fairly important, though not requirements.
* Hard to only hear first then spell correctly.
* Hard to only read first then pronounce confidently.
* Hard to find when searched, even in combination with other likely terms (e.g. video, streaming, app).
* Can be confusing when introduced as a new-ish term in conversation.
* Such a notably 'unique' name can trigger discussion about the name itself which can lead to distraction or delay in communicating the product's value prop (as seen in the first third of their own intro video).
* Even for those who may find it "clever" or "cute" initially, it will likely become annoying over time for those in the company as well as the company's most valuable stakeholders.
Against that, the upsides don't appear (IMHO) to be a good trade-off.
* Short
* Quick to say
* Unique
* Memorable (at least on the verbal dimension, if not written)
* The domain was apparently affordable.