Here's the stats from last season on goals from corners:
West Bromwich Albion 15
Chelsea 11
Arsenal 8
Everton 8
West Ham United 8
Hull City 7
Crystal Palace 6
Swansea City 6
Burnley 6
Tottenham Hotspur 5
Liverpool 5
Watford 5
Southampton 5
Middlesbrough 5
Emirates Marketing Project 4
Leicester City 4
Stoke City 4
Manchester United 3
AFC Bournemouth 2
Sunderland 2
Unfortunately I can't find stats on number of corners, so can't work out the ratio. But overall seems we're bang average - same as Liverpool, slightly better than Utd and City, and fair bit worse than Arsenal and (in particular) Chelsea.
Season before we were joint top on 8 with 4 other teams, including City.
I agree that our corners seem brick and ineffective, when watching us play. But stats indicate that might not be the case overall.