While I admit that I completely forgot about the title screen, there still is a difference that make the signpost art look uncharacteristic. Compare the signpost art and Sonic standing next to it. Sonic's brow is at a sharp angle across his eyes, while on the signpost it's more rounded, giving him a more friendly look, rather than a mischievous one. There's nothing wrong with Sonic looking friendly, but the dimples over emphasize his friendliness in the signpost, while in the title screen art it emphasizes his confidence due to the difference in expression. It's difficult to explain, but with pixel art only a few pixels can completely change a character's expression and the specific way it's drawn on the sign post feels too friendly and innocent, rather than mischievous and confident. I recommend just adjusting a few pixels and maybe removing the dimples and seeing if you can make it look a bit more like Sonic.