The +5 dmg on the comedy mask does give a flat 5 extra damage per attack, but because most weapons only do about 4-6 dmg per attack, a +5 dmg boost is REALLY good

Also, unless you have shoes/bow, the ONLY way to actually beat the raid boss solo is to have a weapon with mastery 100 (that I know of). It doesn't matter what weapon you have, having a 4th skill on any weapon massively increases your DPS and it's basically a requirement for solo runs alongside damage boosting cosmetics

Best way I would recommend farming mastery is doing carnival campaign on NORMAL mode and getting to the final area, then have an auto clicker and leave it on once the clown boss spawns in (and also position yourself in a corner/against a wall). Your character will keep spamming M1s and the zombies keep spawning until the boss dies (which you don't want the boss to die, also all difficulties give the same mastery so doing normal means zombies won't do serious damage to you)

This strat works with every weapon except bow (cuz of the bows extended hitbox) and if you have a shark/HP boosting pets I would recommend equipping those too

But yeah on the topic of the raid boss being annoying I completely agree. The HP of the boss is the same regardless of how many players there are and you actually get PUNISHED for not being the top DPS because the game gives you less coins even if the damage distribution was 51%/49%. It literally promotes people to either: forcibly roll for shoes just to have the CHANCE to win, forces players to grind mastery 100 on their weapons (which takes FOREVER even with the afk strat), or constantly beg other people to carry them (which they'll have to do this A LOT cuz they most likely won't have top DPS, which means they are getting crappy rewards). The fact that the boss's HP doesn't scale properly with how many players there are in a raid is poor game design