Just wanna add my two cents here. First, Sonic is the same weight as Mario, so he's a mid weight. Not really "fairly light". The thing that makes the MU so hard is that Lucas can't catch Sonic. Much of Lucas's kit will only work if the Sonic approaches, which is something a good one will rarely do unless they are getting a punish. Shield cancelling side b is also amazing for baiting things out of Lucas. SH Nair can break side b, yes, but if he cancels side b (or just keeps holding it) and baits out a nair, then he is fast enough to punish the end lag despite it being relatively low. It also sucks for Lucas that Sonic is arguably the hardest character in the game to grab, and Lucas thrives immensely off of grabs. Off stage, tether trump is pretty much your only hope of getting a gimp. Sonic's recovery has a good portion of invincibility on it, so he can snap to the ledge while already invincible, thus preventing edge guarding via attacks.
Currently, I'm actually of the opinion that Sonic is handily Lucas's worst MU. If it were me, I'd much rather do the ditto rather than try to go Lucas against a Sonic in tourney.