In my view, the reason clash works at a low level is not because opponents can't exploit a low time-to-kill, short-range, and poor mobility. Rather, it's due to poor awareness. A clash will always lose a fight where it walks forward on flat ground holding ZR and any opponent sees it. Rather, it plays on getting in position without the enemy team noticing, as was already mentioned. Its huge indirect AOE radius in a mode like tower control, when it gets in position, starts dealing indirect damage and paints the opponent's feet, which can land them in a sticky situation when they are already distracted by fighting your teammates, who can potentially (usually unintentionally) combo off the indirects. Other AOE weapons like other blasters, sloshers, and rollers all play into this as well but with far more effectiveness, and while clash is weaker than all these options, in my view it is definitely easier due to the fire rate at which it can land indirect shots, combined with slow decision-making. If, standing on the tower, you get jumped by a clash at low level and your teammates aren't there to kill the clash, the options to exploit its weakness as a short-range shooter are not as effective, and hence why it racks up kills.
With better awareness, it's harder for the clash to get into the positions it needs to land the shots, and teammates will be in a better position to take it out if it gets lucky enough to get in position, especially considering its poor mobility and inability to fight opponents who know where it is. But that doesn't preclude it from landing indirect shots and forcing the enemy to divert their attention.