For me, it's the contrast between the clean lines of the framing and the pencil-sketch attributes of your line art that makes this look unprofessional. Possibly you went over the lines multiple times to make them thicker and that is why it's happening (in digital art, this is referred to as 'petting the line')? Because of the contrast between the two, it feels like only part of the image was done digitally and makes your art look 'unfinished' in comparison to the frame. If you like the line art the way it is, I'd change the framing so it isn't such a harsh contrast (adding a texture to break up the overwhelming solid lines, or drawing it with the same kind of line art for example). Alternatively, you could try changing the line art to solid, confident strokes to better match the frame you've chosen.