Is it allowed for non- Muslims to visit and enter a masjid if they seek to learn about Islam?

Yes, it is allowed for non-Muslims to enter Masjid if they intend to learn about Islam, and do so respectfully. The delegations of Thaqeef and the Christians of Najraan were allowed by the Prophet Mohammed (ﷺ) to stay in the mosque before any of them became Muslims. There were many benefits offered by doing this: they could hear the speeches and sermons of the Prophet Mohammed (ﷺ) see him praying and reciting Qur’an.[1]

Reference: 

[1] From Fatwa al-Lajnah al-Daa’imah, 6/276