I've played a fair amount of poker in my time. I know the way the game works (although I still suck at it), and I have some theories as to why Telltale might have chosen the characters that they have and what roles they'll play during the game.
Poker is as much about personality as it is about luck; you're playing the people as much as the cards. Now, consider the personalities of the characters involved:
Max is crazy. You'd worry about going into a heads-up with him, because you get the feeling he might just bet on anything just for the sake of it. He'd be a total wildcard, utterly unpredictable. He'd play loosely and aggressively, although not through any kind of tactical knowledge. In fact, with these types of players it can be tough to know if they've got any idea of what they're doing at all.
Strongbad is arrogant. He'd talk a lot, bet heavily and bluff big, while being unlikely to flat-call - he's a born raiser. The ego-driven approach of these players tends to make them fairly predictable, so if you can figure out a tell when they're bluffing then you should be able to beat them.
The Heavy is the strong silent type. He's likely to hold onto his chips until he gets a good hand and then bet big. The rest of the time he'd be a cautious player, playing conservatively and guarding his chips ferociously. It can be difficult to dislodge that kind of player off a hand.
Tycho is a tricky one. In the Penny Arcade comics he comes across as the more sensible of the two characters, so I suspect that on Poker Night he'll be the calmest and most skillful of the opponents. Essentially, he'll take on the role of the Shark, and will probably be the hardest character to beat.
So essentially, each of the characters Telltale have chosen is fairly close to a known poker archetype; Max as a Maniac, Tycho as a Shark, the Heavy as a Rock and Strongbad as a Loose-Agressive type.
I might be overthinking this, and of course Telltale might have chosen their characters for entirely different reasons, but if so the similarities are remarkably close. Any thoughts?