No. No he wasn't. Not in goals scored when playing upfront, which are, after all, the raison d'etre of a striker. The figures are pretty clear on that. A 5/6 week purple patch when he wasn't even playing upfront seems to have convinced a lot of people otherwise. Including you it would seem.
Cullen has been, to be charitable, not very good as a striker for as long as we've tried him there. He wasn't very good four years ago when he had Joel Piroe supporting him and Morgan Whittaker kicking his heels on the bench instead, and he's not very good at it now.
The difference in the way the team has played this season with someone competent leading the line has made the difference so obvious I'm kinda surprised that anyone is arguing to the contrary.