If a crewmate disconnects/quits, are you then automatically screwed on tasks?

If a crewmate disconnects during a game, their remaining tasks are deducted from the task bar. So, yes, if you are the last player with tasks, you could disconnect and have the crewmates win.

Source: personal experience. Had a player disconnect who had quite a few tasks left and noticed the bar was changed and we were able to get a task win shortly thereafter.

Depending on the amount of crewmates left, having a crewmate disconnect can cause different results. If one more crewmate needed to be killed by the impostor to win, the impostor will win the match if the crewmate disconnects (I think the same happens if one crewmate needs to be killed to win and that last crewmate has the last task(s), if the crewmate disconnects the impostor will win instead of the crewmates). Sometimes the opposite happens where the crewmate disconnecting causes the crewmates to win since the tasks become completed.