I'm Mega Man main, yo!
I initially thought that his projectiles were lame because of low damage, limited range and very punishable due to end lag, but figuring out how you can annoy players with them you'll really do good as MM.
His pellets seem weak due to low damage output and short stun, but it can interrupt a lot of attacks: small projectiles, dash attacks, smash attacks. And it can really interrupt those who are trying to recover or getting up after a ledge grab. Learn to utilize it while keeping a safe distance.
Keeping enough distance close enough where your attacks still hit while being far enough to avoid getting hit by dash and tilts are the sweet spots for Mega Man users. From there you can approach with your own dash attack (top spin), dash then up smash (spark attack), a soft charged buster attack (fsmash), or a grab.
Also, crash bomb and metal blade are a good attacks when you are at a farther distance, with Crash Bomb getting at a farther distance. Crash Bomb when successfully attached will force opponents to four things: hold the shield when it explodes, continuous dodging, try to approach you, or just absorb the attack. From there you can plan out what to do.
I'm still not good enough due to my bad habits kicking in sometimes, but those tactics are enough to win me 70% in 500+ FG battles in the Wii U version. If you really want to get into more advanced things, Anonymous Tipster is one Mega Man master here, he can sure give a lot more tips than me