Within those countries, IS supporter are far less than 1/6
I am not that sure. Among the Maghreb nations, only Morocco is having no history of Islamist insurgency.
There was a bloody civil war in Algeria which lasted for more than a decade, where the government forces fought against groups such as the
Islamic Front for Armed Jihad and the
Islamic Salvation Front. The brutality exhibited by the FIS was extreme (even the ISIS wanes in comparison). Also, the Islamic Salvation Front received close to 50% of the votes in the 1991 elections.
And although Tunisia has remained peaceful for many decades now, don't forget that the current ruling party is a local branch of the Islamic Brotherhood.
IMO, the ISIS support might be very low in Morocco, but significant in both Tunisia and Algeria.
As you said Morocco is fine although it got direct threats from Aqmi and Isis hence they've raised the security level to it highest (aka military with army equipment and SAM systems deployed near all major cities and key infrastructure
As for Algeria because of the military insurgence against the Islamic government that won fair and square in the 90s it resulted in terrorist acts and to this day they are still happening, and the population are having enough of this and not to mention that the current government PR is that they are fighting them, hence people in Algeria are very sensible to anything related terrorism
As for Tunisia might a tad bit more liberal and secularist than Morocco, and the Tunisian rulling party has nothing to do with the Islamic brotherhood, the Party is similar to the one rulling in Morocco or in Turkey, while it members has an Islamic orientation, when it comes to ruling the country, they are working as an Political party like any other without mentioning religion ( a mistake that the Muslim brotherhood made in Egypt )
Still 16 % is huge, if you read the PDF, it is split like something 1 or 2% favorable and 14-15% slightly favorable) but the key thing for me in that article is the date of the survey as I mentioned which July and till then ISIS was still presented by some media as fighting the bloody Syrian regime no more no less, I'm pretty things would be different right now.
I feel like that the vox article being posted right now is to serve some agenda or just to criticize and be a click bait especially under the current circumstances