Thats very arguable by looking at results. Took results from Wiki and if I miss some technical or else problem, pardon me.
Germany: Kimi was 2nd in race and 4th in quali, Romain was 3rd in race and 5th in quali. No technical problems in race for either of 2.
Hungary: Kimi was 2nd in race and 6th in quali, Romain was 6th in race and 3rd in quali. No technical problems in race for either of 2. Also Romain received 20 second penalty for causing a collision after race.
Belgium: Kimi retired race for brakes failing, 8th in quali, Romain was 8th in race and 7th in quali.
Italy: Kimi was 11th in quali and 11th in race, Romain was 13th in quali and 8th in race. Race took on 8th of September, and Kimi's Ferrari contract was announced on 11th. We will never find out what kind of impact it had for results, Lotus could have switched their focus more to Grosjean after that.
Singapore: Kimi was 3rd in race and 13th in quali, Romain retired for pneumatics issue and was 3rd in quali. Also Kimi suffered his back injury here during weekend which he later went to get fixed. Also don't remember how far away of Kimi Romain were before his retirement.
Korea: Kimi was 2nd in race and 10th in quali, Romain was 3rd in race and 4th in quali. If I remember right Kimi overtook Romain on track.
Japan: Kimi was 5th in race and 9th in quali, Romain was 3rd in race and 4th in quali. Romain had first corner incident.
India: Kimi was 7th in race and 6th in quali, Romain was 3rd in race and 17th in quali. Kimi lost grip of his tires few laps before finish and was on 2nd place.
Abu Dhabi: Kimi retired race on first lap after colliding with another driver, was 5th in quali but later disqualifyied after failing floor deflection test. Romain was 4th in race and 7th in quali. After Abu Dhabi Kimi went to get his back fixed and was replaced by Kovalainen
Do you still think Grosjean was arguably better driver?. Seems to me that you just don't like Kimi or some other results or performanse unrelated thing
Grosjean was faster in Germany and Korea but SC ruined his strategy both times. In Singapore he lost a podium by mech issues, although Kimi had his share of that in Abu Dhabi.
All in all, Grosjean was better in that period even if it wasn't a big advantage.
He also had the edge in the first half of 2012, but the incidents in Monaco and Australia, also the failure in Valencia costed him massive points.
He utterly sucked between Spa 12 and Britain 13, but that idiotic crash might have been a reason.