While the founding member of the house, Salazar Slytherin would have probably denied entry to a student who was Muggleborn, it is still possible for a Muggleborn to be in Slytherin. There is no specific rule or requirement that claims for only Pure-Bloods or Half-Bloods to be a part of Slytherin. The traits outlined to be the defining features of a Slytherin are very likely to be found even in Muggleborn students.