I think it is fine that it is really weak early (because they want him to be a late game team fight powerhouse or mid-late game roaming assassin carry), but it doesn't scale well enough at all. It deals true damage, which is nice and all, but the damage is so pathetically low that it doesn't matter. dealing 36-216 damage over 6 seconds is really weak considering the amount of attacks it takes to stack, so I would suggest changing it to scale up from 2/3/4/5/6 like live instead to 2/4/6/8/10, so that late game it at least does between 60-360 over 6 seconds (between 10 and 60 damage per second, or an increase of 66.66% damage per second) which rewards twitch for stacking up really high on a single target (and who but a tank should be taking much more than 6 attacks from a late game ADC anyway, except when tagging people during his already semi-underwhelming ultimate), but doesn't significantly increase his lane presence until mid game. If they are concerned with giving him too much objective control (which they shouldn't after the slap in the face that was Kalista) they can cap the damage vs monsters at no more than 6 damage per second per stack, which is how he is currently.
EDIT: To put that in context, at lvl 18 after a full 6 auto attacks on a target, Twitch's passive after this buff would only deal 1/8th of an AA more damage than it does now, which is not that huge of a buff.