AsstEvlHmnRcrsDrctr : get over it. IF you're so concerned with grammar, be an editor in a publishing house. I'm a grammar fanatic, but sadly (I guess) the grammar of comic strips isn't really that interesting to analyze.
But you asked for it... In this case "No." can be a sentence. Because the implied meaning in conversation is "No, I don't need help with my project". It's just truncated for the colloquial purposes of not repeating redundant information when it can be derived from context. Considering this is a comic strip illustrating conversation between two people, and not an essay, that's perfectly fine.