I think we should differentiate between (i) single founders who can't find a cofounder and (ii) single founders who don't need a cofounder.

I think case (i) would be a red flag because it means the single founder sucks at selling herself, sucks at networking, or isn't resourceful enough to move to a location where there are more potential candidates.

In case (ii), where the single founder doesn't need a cofounder, that founder also probably doesn't need YC.