Without getting into COMPLEX arguements over the Pictish culture, some prominant scholars that study the Picts believe that THEY were matrilinial.
Scotland means "Land of the Scots," who were the ones that came over from IRELAND and established "Scottish" Dalraidia as a portion of the older "Kingdom" of Dalraidia. Dalraidia was based, generally, in the modern Argyle region. The Scots were distinct from the Picts.
IF there was matrilinial descent, it would have died out when the Scots "overtook" the Picts (no matter WHAT form that took).
The Pictish kingdom merged with the Scot one under Kenneth MacAlpine (or whatever spelling one gives), who became king of both and "Scotified" the whole. Kenneth did inherit the Pictish title through his maternal links, though such is NOT proof that this practice was standard, unusual, or something in between. In later generations, the "high-kingship"/national monarch was elected from a body of cousins-> all within a set generational descent from a previous king. They were the sons and grandsons of both paternal and maternal lines down from a king. (In the case of Duncan, MacBeth, and Thorfinn, they were cousins of the same generation and Thorfin supported MacBeth as the most qualified candidate, esp. after Duncan's disastous reign and failed combat.) It wasn't until the ascention of Malcolm Canmore (Big Head) that direct paternal descent of the monarchy became standard practice, being adopted by Malcolm following his 17 years in the Anglo-Saxon royal court in England.

At the moment, I forget if, at Macbeth/Duncan's time if it was the grandchild of a king, the great-grandchild of the king, or the great-great grandchild. I THINK it was the great-grandchild. I'll look it up later.

Pictish history, society, and culture is the subject of MUCH debate. If one looks at the Osprey book on the "Pictish Warrior: AD 297-841" (Warrior series, 50), one gets a glimpse at the mass confusion (and the reason for it) when trying to understand the Picts. Simply put, they didn't write down anything we can understand andwe only have fragments of their language (which is not even enough to definatively say if it is P-celtic, q-celtic, or another language completely). It WAS different enough that Colum Chille needed a TRANSLATOR when he first dealt with Brude in what is modern Inverness. What we have is VERY fragmentary primary sources and slighted early secondary material.
The matrilinial lineage as the central lineage is ONE of teh hotly debated issues among modern Pict scholars.