Is it a religious error to refuse to marry a girl from Ta'if because of what her people did to the Prophet Muhammad, may God bless him and grant him peace?

If the woman is religious, do not turn away from her simply because she is from Taif or any other Arab tribe, because Islam abrogates what came before it, and no bearer of burdens will bear the burden of another. Many Companions were from Taif, and the Companions married women from among them. Your acceptance of marriage to her is an act of filial piety towards your mother. There is no legal error in your refusal to marry her for which you would be sinful, as you are not religiously obliged to marry someone you do not desire.