Mario
Skills
Jump Attack- Decent damage skill with a minor stun at the end. Only complaint I can honestly say is the deselecting that happens when it is used. (Same thing with most jumping skills) Its a minor inconvenience at best. Goombario's Jump attack also makes Mario a bit more micro heavy than most of the crew (if your aiming for maximum effectiveness, not as micro heavy as a well played robotnik but you get the idea.) I wouldn't change anything here.
Star Power- Here's a fantastic move for Mario to use during heavy boss aoe attacks and to get some free damage in. Keeps Mario healthy through otherwise lethal damage. My only suggestion is leaving the duration as is, but reducing the cooldown for the last rank a bit (perhaps 15 sec invulnerability every minute (raise the mana cost if you must).
Fire Balls- Now this is where a debate can definitely brew. From my honest opinion, fireballs rock. There is a minor detail wrong with the tool tip (30 mana listed but it really only costs 8). At the early game a fireball heavy Mario will be incredibly mana intensive, but his added damage will definitely make up for Mario's slow attack animation. Fireballs appear to be adding a significant amount of damage to Mario's attack (he starts one hitting Goomba's at level 2 with no additional items involved.) The only argument here is the other spell casters that will argue with Mario over the wizard ring

(No complaints here)
Firestorm- Mario's bread and butter skill. This is the ability that makes Mario such great support during the game. A massive aoe stun ability that stuns for as long as Mario gets a clean channel. Its a fantastic ability and there really is nothing else to say about it. The cost is perfect and the cool down is set so that you just can't chain stun bosses to death.
Super Mushroom- I like the idea of giving Mario a spell immune form to help him tank, but a part of me thinks that this ability feels more like a placeholder than anything else. Sure Mario gains some HP and spell immune, but it feels like its incredibly lackluster when you compare Mario's ultimate to say X's ultimate. A suggestion that I like is to make it so Mario's ultimate is the hammer bros suit. He can gain his spell immunity but allow him to throw ranged hammers for the duration with an attack speed increase and a damage boost. Plus its a nice throwback to Super Mario Bros 3 (one of Mario's greatest games.) You can let Luigi keep his ultimate .
Items
Mario's Hat- Perhaps listing the amount of increased attack speed and movement speed would help people make a choice of whether to keep it or to chuck it for later weapons. Right now, the attack speed boost is useful, not really noticing a major move speed bonus.
Feather Cape- Ah a classic power up item with a slightly lackluster ability. You give the cape the bonus of improving Mario's ability to take spell damage and it blocks a spell as well, but this item usually gets replaced by a 45 d item or an ultimate sword (depending on a tanking Mario or a damage dealing one) The cape gets worn for just a little bit. (most class items in the game are usually best in slot items for that character , same should be for Mario.) Maybe make the cape have the ability to reflect a spell cast at Mario (or easier to code, make Mario spell immune with it on. You could also give it a movement speed bonus.
Mega Hammer- Honestly it seems people would skip this and just opt for the ultimate sword in its place. (maybe but a 5-10% chance to stun his enemy?)
Tanooki Suit- While I enjoy this throwback to SMB3, I think the suit could be improved by applying a taunt to it much like Kirby's rock form. This could make Mario a fantastic late game tank.
Goombario-
Mushrooms- best ability this little bugger has! Not related to the game of his spawn, but he does help support the VG team with near unlimited health items. No complaints here.
Headbonk- Does the damage a sidekick should do. Its added damage to being able to micromanage Mario successfully.
Goombario's Cap- A completely overpowering item and so early. Makes Goombario do more auto damage then some people heading into stage 2 (near full game of course!)
Goombario's usefulness comes from the fact that he can be given aura items and not have slots wasted on Mario. He essentially becomes the aura bank. Other Goombario builds could see him stacked with his cap and 5 ultimate swords for a nasty damage dealer.
Items aside, Mario has very solid moves. His main hat gives him a huge AS boost and a minor MS boost. I honestly think if you wish to keep the Super Mushroom ultimate, at least make Mario's armor divine for the duration. If you wish to go the hammer suit route, it would give Mario a much needed ranged attack if he wished to be a damage deal and he could tank like a champion with it anyway. Overall I think Mario's items need the most work here.