Slavic women considered ugly? That’s the first time I hear of this, for any time period and region. While in many cases they were considered rather attractive. Heck, the early Arabs even thought all European women were seductive blondes and highly promiscuous (respectively, many Slavs, both men and women, were sold as household slaves to the Arabs back then and were rather highly prized (along with Turkic slave soldiers)). Though, who knows, maybe the Mongols just had bad taste.