Why didn't Sirius Black use Polyjuice Potion to escort Harry to Platform 9¾. Instead he transformed into his dog form. Also he could have used the potion to get out and about, away from Grimmauld Place once in awhile. It is a good disguise, why didn't he ever use Polyjuice Potion in the entire series? Did he think of this and Dumbledore did not approve? Did the Order members think it was too risky? It seems less risky than going out in his dog form, which Pettigrew knew about and told the other Death Eaters and Voldemort about.
The Polyjuice Potion, which is a complex and time-consuming concoction, is best left to highly skilled witches and wizards.
—"Polyjuice Potion", Pottermore, by J.K. Rowling (emphasis mine)
Being an Animagus, Sirius Black had an instantly available method of disguise. Contrast that with Polyjuice Potion, which is hard to make.
The effect of the potion is only temporary, and depending on how well it has been brewed, may last anything from between ten minutes and twelve hours.
—"Polyjuice Potion", Pottermore, by J.K. Rowling
Not only is it difficult to make, but the duration of effect is unreliable. Meanwhile, an Animagus can remain in their animal form indefinitely. Peter Petigrew remained disguised as Scabbers for several years.
Polyjuice Potion makes one turn into someone else. On platform 9¾, that would need to be someone who wouldn't be out of place on when parents are seeing their children off to school. The Wizarding world is quite small and all parents would certainly know each other. Sirius would have needed to disguise himself as someone close enough to Harry to not raise suspicion, yet not so close that they'd want to be there themselves.
But who pays attention to the family dog?
The four [Marauders] were viewed (by themselves at least) as the rock stars of Hogwarts. [...] Sirius was a good-looking rebel [...]
—"Everything we know about the lives of the Marauders", Pottermore, by J.K. Rowling
Sirius sees himself as a rock star rebel, with his motorbike and his good looks. Turning into Padfoot is like a signature move to him. No doubt he thinks himself a handsome dog too. While he is in disguise, he's still proudly being his own self.
Using Polyjuice Potion to disguise himself as someone else is not in his nature. Yes, it would be safer, but it wouldn't be like Sirius. Sirius liked taking risks.
[...] Sirius added, "When's your next Hogsmeade weekend, anyway? I was thinking, we got away with the dog disguise at the station, didn't we? I thought I could—"
"NO!" said Harry and Hermione together, very loudly.
"You're less like your father than I thought," he said finally, a definite coolness in his voice. "The risk would've been what made it fun for James."
—Harry Potter and the Order of the Phoenix, chapter 14 "Percy and Padfoot", by J.K. Rowling
So while it would've been a bit safer, it would've been a lot less practical. Sirius was an unregistered Animagus, so he probably thought himself quite safe to begin with. And it just wasn't like him to use Polyjuice Potion to hide.
Of course he would turn into Padfoot instead of using anything else.