How do I make use of charizard's tilts effectively?
He's floaty with a crummy airspeed so its difficult to airdodge a landing into a tilt the way squirtle and wario can.
He is the slowest walking character in the game and its difficult to dash into tilts without being obvious.
His movement is just so awkward.
A lot of times, I say, "Tilt like you're DK" against characters with bad range, except D-Tilt isn't quite as safe. Regardless, abuse the range, and don't be obvious with them if you can help it. The important thing is that they have excellent range; it's worth making good use of it.
If someone is above you, U-Tilt will probably beat out whatever they have. That's probably his best tilt, and is the most spammable, for sure. F-Tilt is for when you feel they're going to do a grounded move that doesn't have much range, or if they like shorthopping a bunch (make sure to angle it upward there). Remember that the D-Tilt causes Charizard to lean back, so you can avoid some moves that way and retaliate, but it isn't particularly reliable. Jab -> Jab -> D-Tilt works at middling percents.
Charizard's run speed is 8th best (barely slower than Meta Knight). Charizard's ground game would've been amazing if he didn't stick his neck out while running, because it means you really can't dash up to anything offensive outside of instant B moves and RAR'd B-Air. On many good characters, you can't even dash, shield an inevitable attack, and expect to grab them, because he moves his neck backward when he shields, mostly negating the range advantage he would have in that situation; all it does is give people an opportunity to hit you before you start anything. This is why Rock Smash and (especially) Flamethrower are so important--They give you fast, safe, useful options out of a run, and condition your opponent to worry about them instead of your other moves.
Also, Charizard's grab is a pain, because it doesn't start at the base of his body, and people who get up close and personal (like, say, Falco) can avoid grabs just by being on top of you. Use shorthop N-Air, fullhop D-Air, or D-Smash to prevent it, depending on how safe they're being.