Somewhat.
Comparting his production numbers to other rookie WR's is misleading - JSN is a clear #3 WR on a team that is targeting quick passes due to injuries, and heavy TE involvement due to frequent 2-3 TE sets. So, his numbers aren't real concerning to me, and I'm not worried that his production is way behind some other rookie WRs.
With that said, I have been a bit concerned about his actual performance. He doesn't have any chemistry with Geno, and balls thrown his way often look awful. He hasn't been elusive or strong with the ball in his hands, and hasn't made the most of some limited options down the field (i.e. the drop last week).
It's early, and WR is historically a very difficult position to transition into the NFL. But, I do confess that I'm less optimistic about him than I was three months ago.