I find Sonic vs Little Mac to be an extremely favorable match up for Sonic.
Sonic controls the entire flow of the match with his approach options as long as he is outside of Little Mac's FTilt and somtimes FSmash range. Little Mac also has an incredibly terrible aerial game and basically cannot punish Sonic for SC/SD approaches. Also, Little Mac's counter is ineffective against SC/SD, not connecting most of the time. A single mistimed smash attack / Ftilt / Counter from Little Mac to attempt to hit Sonic out of SC/SD will result in SC/SD connecting for a ton of damage and possibly put Little Mac offstage where he is incredibly easy to gimp thanks to the spring and Sonic's solid aerials. Little Mac's best option for stuffing SC/SD is his Jab attack which Sonic can easily shield/jump cancel to return back to the neutral game. Sonic's aerial SC/SD also threaten's Little Mac's attempts to punish Sonic as he lands or simply leads to another favorable mix up for Sonic.
One of Sonic's best tools, especially against Little Mac, is to keep him guessing which direction and how far you will run with your Sonic Speed. If he FSmashes/Ftilts towards you, depending on your distance and timing, you can simply run away forcing him to whiff and punish with reverse SD ,FSmash or FTilt. This leads to an unfavorable mix up for Little Mac. Little Mac is forced to guess your retreating run. If he guesses the Sonic's run is too short and does a dash attack, he will whiff. If he guesses at the right spot and does a dash attack, Sonic can simply reverse SD (the sound will likely force the Little Mac to try to hit you out of it) and shield on reaction to Little Mac running towards you and punish. If he does a grab, you can outrun this even more easily or spot him coming with ample time to spot dodge/roll. If Little Mac decides to advance towards you and shield, he will then need to try to punish the SD or guess if you will cancel and grab/attack/jump instead.
Assuming both players are good and keep mixing these mix ups up and Sonic and Little Mac trade hits, Sonic's Spin Dashes and Grabs lead to multiple follow ups of which Little Mac has few tools to do anything about and often put Little Mac offstage at relatively low %'s. Whereas when Sonic gets hit by Little Mac, Sonic has many options which threaten / nullify Little Mac's follow ups.