Toon Link is by far one of Lucina's worst matchups. She's good vs. Rosalina though, it's probably an even mu or maybe slightly in Lucina's favor - the sword makes it pretty easy to kill Luma. I would say Mario and Fox are roughly even, maybe slight disadvantage to Lucina.
Not sure about picking up Lucina as a generalist secondary. On the one hand, she has a pretty high floor - she's easy to pick up and use at a decent level. On the other hand, she has a decent number of bad matchups, so it might be worth using someone who's a little higher on the tier list. (I would recommend her over Marth as a secondary - Marth requires a lot of precision and training to be superior to her.) Some easy higher-tier options might be Diddy, Mario, or Cloud.
Main reason I'd recommend her as a secondary would be because she covers some high-tier matchups well, like Ryu, Rosalina, Yoshi, and especially Ness. But yeah, maybe not if you just want someone to fall back on.