When the narrative was "Thiel's connection to YC is about prestige, so YC should sever that connection to preserve its reputation", I found it basically reasonable.
But if the narrative is "Disavowing Thiel will make Trump-supporting founders uncomfortable with coming to YC, or uncomfortable admitting their support?" Then we've jumped to punishing people professionally for being in a group comprising 40%+ of the country.
Hounding founders who support Trump (who, perhaps, don't even donate or like many of his views) seems to go straight into the awful kind of patronage/exclusion politics that's being condemned here.
I'm not trying to be uncharitable here - I honestly don't follow. One possibility is that since YC founders often end up quite wealthy, you're talking about discouraging the next Thiel rather than discouraging business with Trump supporters in general?
To be clear, I am not a Trump supporter, but dehumanizing Trump supporters is not a productive approach.
Cutting people off only further isolates them. If you really want to convert Trump supporters, you have to show them the error of their ways gradually, just like with everything else. Writing them off as irredeemable based on their political preferences is not only inadvisable, but fundamentally destructive of the binding social fabric of the republic.