Why would you not take him seriously?
That said he is the symptom not the disease and that we absolutely should take seriously.
Even if it doesn't come to pass the trajectory the US is on is possible enough we have to consider it.
Not taking him seriously already cost the Democrats two presidential terms.
He has the charisma of a mouldy grapefruit. It certainly wasn't his charisma.
He was elected because he was able to convince many, many people that there are simple solutions to complex problems and that he was the only person in the world "brave" enough to implement them.