Short answer - yes.
I can imagine circumstances where this would seem wrong, and also circumstances where it would seem right.
From experience, a poll took place to fill two vacancies. Six candidates stood. Then another councillor resigned, and the person who came third in the poll was co-opted. That seemed the fairest solution and respected the views of the electorate.
On the other hand, if the co-option is flying in the face of electors' views, it would seem wrong. But the only reliable solution is at the ballot box!