Huck's grammar is substandard. He uses such terms as "ain't", incorrect verb tense such as "I seen" and incorrect verbs such as "she learned me" and "I set down." He says things such as "Tom and me" instead of "Tom and I." He uses a lot of slang expressions such as "victuals" and "I lit out" and other vocabulary that is probably part of his regional dialect, i.e. "it was an awful sight of money" and "he was no kin to her." So, quite early on, we learn from Huck's speech that he is young, poor, rural and uneducated. That is part of the charm of the dialogue, however.
As far as whether or not it makes him a reliable narrator, my view is that it lets us know that while he may be young and uneducated, he is also honest and perceptive. Just because one is uneducated does not mean one is unintelligent. As the novel progresses, we see that Huck perceives things much more clearly than even he realizes. His keen sense of perception, I believe, indicates that he is going to be a very reliable narrator because he sees things in his innocence and remarks upon them in sort of an "out of the mouth of babes" sort of way. For example, in this first chapter, Huck describes how he had to get away from the Widow Douglas and her attempts to "sivilize him":
They get down on a thing when they don't know nothing about it. Here she was a-bothering about Moses, which was no kin to her, and no use to anybody, being gone, you see, yet finding a power of fault with me for doing a thing that had some good in it. And she took snuff, too; of course that was all right, because she done it herself.
He perceives that the Widow, although good hearted, is a bit of a hypocrite.
Read about the novel here on enotes.