Why does Matthew Pocket tell Pip he isn’t “an alarming personage” & what Pips impression of Matthew?

By describing himself as not being an "alarming personage", Matthew Pocket is referring to the fact that he isn't anyone to be afraid of. Pip found Mr. Pocket to be natural, genuine, and unaffected, as well as comical.

He was a young-looking man, in spite of his perplexities and his very gray hair, and his manner seemed quite natural. I use the word natural, in the sense of its being unaffected; there was something comic in his distraught way, as though it would have been downright ludicrous but for his own perception that it was very near being so.