No, Tom could not have defeated Sauron. As Gandalf mentioned, Tom could not use the Ring's power - rather the Ring did not affect him.
'It seems that he has a power even over the Ring.' 'No, I should not put it so,' said Gandalf. `Say rather that the Ring has no power over him. He is his own master. But he cannot alter the Ring itself, nor break its power over others.
Galdor and Glorfindel then describe Bombadil as not being able to withstand Sauron in the end, once Gandalf suggests he won't lead the use of the Ring:
'But in any case,' said Glorfindel, `to send the Ring to him would only postpone the day of
evil. He is far away. We could not now take it back to him, unguessed, unmarked by any spy. And even if we could, soon or late the Lord of the Rings would learn of its hiding place and would bend all his power towards it. Could that power be defied by Bombadil alone? I think not. I think that in the end, if all else is conquered, Bombadil will fall, Last as he was First; and then Night will come.'
'I know little of Iarwain save the name,' said Galdor; 'but Glorfindel, I think, is right.
Power to defy our Enemy is not in him, unless such power is in the earth itself. And yet we see that Sauron can torture and destroy the very hills. What power still remains lies with us, here in Imladris, or with Cirdan at the Havens, or in Lórien. But have they the strength, have we here the strength to withstand the Enemy, the coming of Sauron at the last, when all else is overthrown?'
Both quotes from the Fellowship of the Ring.