It absolutely is a relevant excuse - we have played 17 games more than Brighton and Newcastle, 11 more than Liverpool, more than city even. Yes top teams play more games... But we do not have a top squad, are not a top team, but are thrown into it because we played really well earlier and now are barely surviving through it. How do you think Newcastle or Arsenal or Brighton would fair with needing to play this amount of fixtures? Get rid of all their mid week breaks since September (which is what we've had to deal with) and you would see massively different performance levels from them, rather than having full weeks to train and recover and prep for matches.
We are very clearly physically drained, it shows every 2nd half, there is not a more clear factor than fatigue if performance levels drop after half time consistently. We don't have the squad or starting 11 to handle it. We are competing against other quality sides in the prem who have played 15-20 games fewer this season. And it shows in this part of the season.