Everybody who is saying that the non-pro player is a "n00b" are quite mistaken. Pay attention to the SECOND word in the OP's post. He said mediocre, not a noob. Hell, I'd consider myself hardly above mediocre, but I certainly know my starman from my bunny ears.
And to everybody claiming the pro would win - The star disagrees.
^^THIS!!!
However the scenario is still far to uncertain with many variables still unconsidered. And the fact that the OP said that for variety we can discuss about this scenario in any of the smash games makes it more difficult.
Like which characters they are using (It does matter.)
Which stage they are on (TOTALLY matters.)
Also people will have a different definition of mediocre.
Also, I have a question? Does the person with the ability to create items have ANY limit?(Besides the smashball.)
What I mean is, can you only create one item for the entire match? One type of item for the entire match? Lose this power once the opponent is on their last life? Summon only one of each item? Only have one item and it's effects on the screen/stage at any single moment? Only can use this power if he's losing? Etc...
If he truly can summon any item besides smashballs, then yeah, he's invincible (Some items are crazy cheap. Especially in combination with others.) This would be far from epic, and boring since it wasn't even a glimmering hope.
If there were restrictions on his ability (The 'have only one item and it's effects on the screen/stage at any single moment' is my favorite, as he couldn't use all items in one match.) then we're talking about a VERY VERY interesting game. Disregarding stage, characters, and how good the player with the ability actually is, the pro can beat him if he takes advantage of the limit in his ability. However if there is none, then there is no chance, no point.
This sounds like an awesome hack code. (Allowing one player to have the options of the training mode in a real match. :D XD)