Is it permissible to propose to someone whom you reject due to not finding their appearance acceptable, without even sitting with them, even with Istikhara?

It is from the Sunnah to pray Salatul-Istikhara (the Prayer for Guidance) when contemplating a significant matter, such as marriage. The Prophet, peace and blessings be upon him, used to teach us Istikhara for all matters. Therefore, do not decide on marriage except after consulting those of sound judgment and performing Istikhara. Whatever Allah decrees thereafter is good. It is not a condition to see the person for whom Istikhara is performed.