Nah. If you're saying "a main event match at Mania", and not 'the main event match at Mania", then it's not ludicrously uncommon.

Orton/Batista/Bryan was WM30's main event.
At the previous PPV, '14 Elimination Chamber, the main event was Orton retaining by last eliminating Bryan in the Elimination Chamber match. So, technically you could say the last guy is Daniel Bryan (though it's a multi-man match).

'Taker/CM Punk was one of the 'triple main events' at WM29.
At the previous PPV, '13 Elimination Chamber, Punk lost to the The Rock in the main event. So, technically you could say it's CM Punk (though their 'main event' was the third-last match of WM29, but whatever).

'Taker/HBK was the main event of WM26.
At the previous PPV, '10 Elimination Chamber, 'Taker lost the title in the main event Elimination Chamber match to Jericho.

There's more floating around, too (Cena lost the title at No Way Out '09 in the main event Elimination Chamber match, wins it back at WM25), especially depending on how loose you want to take it.