Well, I'd personally recommend that you know how to catch items with the grab button. I feel too many people just try to avoid my grenades altogether, which is about half of his metagame in favour (reading their defensive options, I suppose). A good Banjo will look to combo the explosion into Side B or Fair for confirms, but I doubt that many caught up to it. Just beware.
And that reminds me, Side B. This move means you never really want to throw out moves into the air if he can reach you. The same goes for catching your landings by reading either airdodges, or attacks. You might sort of want to dash in and shield every so often, so he doesn't randomly hit you. If you have a very ofensive character constantly on his face, than you can shut it down before startup (moves and invincibility start on Frame 18, so you have a shot).
Some of his normals are deceptively good, like FTilt, Dtilt, Bair and Nair. Ftilt has really low cooldown, and it's really safe if they space it well. DTilt has massive range, low profiles and easily sends you into tumble (tech situation). Both of these two, 2 frame. Nair is kind of weird. Doesn't do a whole lot of damage unless all hits connect, but then it's this weird, constant hitbox that seems to beat out a lot of things (it's some weird spinning disjoint or something). Banjo can dragdown Nair you for a pseudo chaingrab into U-Smash (catches you if you mash slow) or UTilt (if you mash out fast, catches jump). Bair is suprisingly good OoS, catches your jumps, but it's pretty punishable if he doesn't space it.
Lastly, it's probably his recovery. People might not have fully optimized him and might go for a very linear low recovery or spend Side B really quickly. A friend says someone good at edgeguarding him will likely have him spend those a lot, but I personally think a good Banjo will more often go for the grenade recovery (which can also be punishable