Why are you using sideB so much? If things/chars easily beat it out, then use something else. Squirtle is extremely mobile. You should not be depending on Withdraw as your only way to get in, and what about your ability to zone with WG and Bubble?
Fox. Nuff Said.
Falco. Nuff said.
Because Squirt is so small, he can dash and attack under a lot of lasers. On top of that, SS > pivot JC grab works really on FFers at low-mid %'s. At higher %'s, it leads to great TCs and they usually don't tech it. Their recoveries are
really susceptible to Bubble. You can just do it over and over again until they die. Our bair is also pretty good for knocking them out of upB since our tail has invincibility during it.
Rest of melee top tiers lol.
Sheik seems like she could be a dumb MU. Her mobility rivals ours and our falling speed and weight makes us optimal combo food for her crap. Jiggs is a totally different story, though. I've faced a couple Jiggs with Squirt, including a
really campy one and the MU is probably around even. Squirt can
directly challenge Puff's bair with his own because of tail invincibility. He can counter her ledge planking with WG. You just have to play patient and don't rush anything. Squirt can also kill Jiggs really early with like,
anything. Since she's light and floaty, usmash is probably our best KO move against her, but fsmash works, fair and bair work, and dthrow works.
Charizard: outranges him with nair, can grab him out of side-b and tilt him out of side-b (dtitl, ftilt). Squirtle lacks approach options.
See, why are you trying to approach so much with Withdraw? It's a good move, but it's not an end-all-be-all approach tool and it's beaten out by a LOT of things. You should be weaving in and out with your great ground and air mobility and spacing with moves like Bubble. This can literally be applied to almost all of the MUs you listed that you're having trouble with.
He can be outspaced really well and he dies very early. Still though, I love him, and even though he has a few impossible match ups I will still play him.
'Out-mobilize' the opponent. Yes, Squirt dies early. That's why you want to look for openings and avoid trades. Use your zoning and spacing tools. Abuse that tail!
I feel like its worth noting that if I feel too much pressure in a match up I tend to space with watergun. The method I use is to just keep them at the perfect distance where watergun will hurt them and do small hitstun. A distance where neither of you can actually hit each other.
Personally, I think if you're being pressured too much, Bubble would be a safer and more effective option than WG. WG seems more like a wall/zoning tool. Bubble is fast and can quickly flip the tables.
Its also worth noting that a forward-moving aerial watergun with very little charge can be landcanceled and you can then ftilt very fast. Works wonders against characters with lightweght that wont immediately hit the ground after watergun hits (aka they cant shield right after) like marth, peach, zss, Ivysaur, etc.
Try aerial uncharged WG > grab as well. It's very similar to Sheik's land-cancelled aerial needles > grab.