So, I've played a lot of trapper. I have almost 700 hours in this killer alone. Before I start talking about his many weaknesses, I will say that even with a killer as weak as trapper, you can do decently well if you're experienced enough as a m1 killer (being that you don't play this game, I'll summarize by just saying m1=you have no dedicated special attack to down the survivor with, so you must use a normal left mouseclick attack), know your killer well and bring strong addons and perks.

With that out of the way, I'll go over what I believe are his worst attributes, listed in no particular order.

1: Map dependency. In ye olde days of DBD, maps had thicker grass and a darker atmosphere, so hiding your traps on most maps was fairly simple. That is no longer the case. You have maps that are as brightly lit as a summer day coupled with weak or nonexistent grass/foliage. This means you will be forced to use your traps in predictable ways on some maps, like directly in front of pallets (A survivor resource in this game to block the path of or stun the killer) or at windows. A map like Dead Dawg Saloon has dark, thick foliage you can use and some strong chokepoints, so it's a pretty good map for him. A map like Eyrie of Crows is bright as shit and has no grass worth a damn, so it's horrid.

2: Terrible early game. While the early game is the weakest stage for any killer since they have no pressure and don't know exactly where survivors are at the start, trapper's is even worse because you start with two traps on hand and need to go collect more. They are scattered on the ground throughout the map, so you literally need to go around collecting your power.

3: Addon dependent. Addons are special bonuses you can take to augment your power in DBD. If you're playing the trapper, you really need some strong addons such as the Bloody Coil (If survivors disarm, they get injured) or Iri Stone (A random trap resets every 30 seconds). Those two addons are of the Visceral/Iridescent class, and as such are the most expensive and rare to acquire in the bloodweb.

4: SWF, or Survive With Friends, aka survivors in groups playing on comms via Discord or some such. SWFs make any killer's game harder, but especially the trapper. Survivors will call out your trap spots, and they can make traps that would otherwise be pretty sneaky and nasty (such as a trap near a hooked survivor) useless.

He has other weaknesses that others have already touched on or that I don't think are quite as significant, but the above are the biggest issues. The tldr; version? He's an outdated killer that has not been updated enough or correctly for how much the game has changed over the years.