Does "needless to say" convey to the reader that he is ignorant?

Isn't the term a bit condescending?

It could be construed that way, but it depends on what is said after it. If I said:

Needless to say, two plus two is four

you wouldn't find it condescending, because generally it is needless to say that two plus two is four.

If I said:

Needless to say, cribellate spiders have no need for glands producing hygroscopic adhesive.

You might, depending on your knowledge of spiders, disagree with the first clause and thus find it a little condescending.

It is up to you to judge your potential audience and tailor your writing to them.

No, it just indicates that the writer expects the fact that follows to not be surprising to the reader. The Cambridge Dictionary Online describes the phrase as:

Needless to say: as you would expect; added to, or used to introduce, a remark giving information which is expected and not surprising.

Examples

1) The life and adventures of Martín Chuzzlerwit (Charles Dickens, 1844)

It has been rumoured, and it is needless to say the rumour originated in the same base quarters , that a certain male Cbuzzlewit, whose birth must be admitted to be involved in some obscurity, was of very mean and low descent.

2) The Complete Tales of Uncle Remus (Joel Chandler Harris, Richard Chase, 2002)

No typographical device could adequately describe Daddy Jack's imitation of the flushing of a covey of partridges, or quail; but it is needless to say that it made its impression upon the little boy.

I find it is quite the opposite. If you were to neglect to qualify an obvious statement with something like "needless to say", or "as you know", the reader might then feel like he is being condescended to because you are telling him/her information he/she already knows. By adding the qualifier, you establish some common ground, in essence saying "I know that you know what I'm telling you already, and I don't want to insult your intelligence, so please don't be offended."