To put things simply. Yes. Ragnar Lothbrok was probably a historical person who sacked Paris in 845 AD. It is possible that historical aspects of Ragnar Sigurdsson were later incorporated in the story. However, the story of Ragnar Lothbrok is probably quite far from the historical person, especially the one depicted in Vikings which deviates even from older fictional accounts of Ragnar.

In so far as historical sources go, he is mentioned in the Anglo-Saxon Chronicle and Gesta Danorum (albeit very briefly in both, and do note that these two sources were written some 4-500 years later) the story told in Vikings however is mostly a modernized version of Ragnars saga Loðbrókar which is a sort of follow-up to Völsungasagan.

To add to this question, was Ragnar Danish, Norwegian or Swedish?