Yes, there should definitely be visual and auditory warnings as well when MCAS is engaged. When the autopilot is doing something so important as struggling to prevent a stall (and in its mind, failing, because of the faulty AoA sensor), it definitely needs to be raising alarms.
Also, given how important the AoA data apparently is, it may not be displayed prominently enough. In clear weather, level flight flying, it should be really obvious if this is drastically wrong.
