Tunnelling is when a killer fixates on a single survivor, often chasing them straight off the hook rather than spreading pressure across the team. It is efficient - removing one player early cripples the team - and it is widely disliked in casual play because it ends someone's match quickly.
It is a strategy question rather than a rules question. The game does not forbid it; different communities and rulesets treat it differently.
In competitive 4v1 the discussion changes shape entirely, because results are judged against a standard rather than by feel. What matters is the hook and kill count relative to what the killer being played is expected to achieve, not the route taken to get there.