The Sweden international believes the scoreline flattered the Ligue 1 champions in an intimidating atmosphere in Greece
Vladimir Weiss cancelled out Edinson Cavani's goal when he restored parity for the hosts midway through the first half, but a Thiago Motta double and a late header from Marquinhos saw the Ligue 1 champions emerge victorious in the end.
Yet despite the convincing nature of the scoreline, Ibrahimovic insists his side were made to work for their win.
"It may have seemed easy to get the win but it wasn't. The Olympiakos fans created an amazing atmosphere and this was a great advantage for them in the first half," the Sweden international told Uefa.com.
"Our goal is to advance to the next stage and we got three very important points that set us on our way to qualification. Whether we finish top of the group or not is not really important.
"All the games are difficult in this competition and we cannot afford to underestimate any opponent."
PSG resume Ligue 1 action at home against title-rivals Monaco on Sunday.