I would say the opposite. It seems to me that most fans have pretty tempered expectations for Maatta, especially given his injury. Defenseman able to come in and handle the game as teenagers are much more rare than forwards. He pretty much blew everyone away as far as what he was able to accomplish last year. I don't think many people are expecting to be blown away again.
That said, his success was rooted in his maturity. He seemed to be able to come in, adapt to the level of play, and make small, but meaningful strides to improve his game throughout the season. This is the kind of steady, incremental development path I would project from him (rather than marked highs and lows).
He slowed down at the end of last year because of the mental and physical wear of a long season. This is to be expected, and something that he can be better prepared for in year 2.
I don't expect a sophmore slump at all, tbh. Though if it happens, it happens. Essentially, he's Orpik's replacement in the top 4. I believe he'll be better this year than Orpik was last year.