Those figures are misleading as they've clearly counted all the votes cast rather than counting each party only once in each ward they've contested. See here for an example of how that would skew the result
Although we talk about changes in vote share, the real problem for Plaid is that a lot of their Assembly voters jump ship come local elections and especially at Westminster. Case in point, they got 3000 fewer voters in 2015 than 2016 despite an increased overall turnout of 10000. For Plaid, this election in Ynys Mon is less about winning over new voters than just convincing people who usually vote Plaid to not abandon them once again.