Well, actually historians are kind of split on this one. I don't mean that they don't think that he wasn't great, but some feel that he was dissapointing, that he could have done mucho more. Let me explain:
On one hand, he defended the true and original principles of the Constitution and the economic and civil liberties on which the Revolution was fought for. But, he was considered a disappointment because he didn't determine a path towards a centralized, intervensionist state, he also didn't seem to carry on the same Revolution he helped to originate.
There's so much more to Jefferson's presidency that we can considered great (like his debt reduction plan) to be put on a small paragraph, but I hope this helps!
