«Trump's brand of 'low-brow' was definitely incompatible for a non-zero amount of people with perfectly 'low-brow' tastes»

Probably you are thinking of "middle brow" tastes.

But anyhow I can imagine many "low brow" disagreeing with him or not liking his specific way of being "low brow", but the question was "what was so bad", that is why the reactions of extreme disgust as to the person and extreme rejection as to the politics.

For the extreme rejection as to the person I think it was a feeling of personal humiliation by "high brow" and "middle brow" people as to having a "low brow" "orange baboon" as their superior, because he did not get that extreme reaction when he was just a "celebrity".