It's been a while since I played this MU in tournament, but I used to regularly play vs a local Falco with Mario. Here's what I think...
If you are going to Fireball at all, make sure you full hop it. Otherwise, Falco can just eat right through it with a lot of moves, or just reflect it back at you. Do not waste time short hopping them.
If you get him into the air, chase him down hard, and at the least, camp his landing. Usmash will beat all of his aerials (If you time it right, it should even eat through Bair, but it's tough), and all of our aerials will chain nicely on him if you can connect one to start a chain. Basically, pressure him in the air; you can get nice damage off of it. Getting him offstage is the icing on the cake.
Learn to DI his full jab so you can punish it (uair and up B work nicely). If jab 2 hits your shield, I believe you can grab falco before the third hit connects.
If you get CG'd and spiked at around 35%ish, DI the spike or else you will get placed into a 50/50 situation where if Falco guesses right, you lose your stock. (Mario's only options are 1) Jump and Up-B or 2) Jump, cape, then Up-B. You have to use the middair jump to meteor cancel since UpB won't reach the ledge by itself.) This cost me a tournament set once, so make sure you don't get sent offstage from the Dair.
I think I'd recommend camping at low % too...the risk reward ratio is too far in Falco's favor at low damage IMO. A grab is a lot of damage and gives Falco a lot of momentum.
I have a tournament this saturday, so I'll see if I can run through the MU again to get some fresh ideas. I do know I can say -3 is a stretch. But -2 sounds reasonable since Falco's got the advantage in close quarters, long quarters, and he just gets more reward in general. Mario can do some nasty things once Falco is in the air or offstage, but getting him there without taking too much % yourself is a problem.
Does anyone have an idea of the frame data on Falco's jab 1 on hit for us? Can we Up B or anything? Or do we always get grabbed if he buffers?