No he's not viable imo, can he pubstomp normal queues? sure, any AD can.
can he do it competitively against equal skill level players? less likely.
Leona, Taric, Lulu are nightmares for a twitch. Twitch can only win trades on ashe and trist at early elvels in lane phase.
Ezreal, Cait and Draven flat out destroy him, coupled them with any of the 3 supports above and twitch is 3-4shotted easily,
twitch is a very offensive oriented champ, sadly the only thing that allows him to be that way is his stealth, which is easily countered by a pink ward thats easily affordable
not to mention any hard cc which currently most supports have.
Tbh if you want an agressive ADC in lane go MF or Draven, if you want one that can sit back and farm abit and poke/commit at good oppertunities, go trist/caitlyn.
Twitch is offensive but is forced to be very defensive just based on his easily countered mechanic.
dont expect to creep behind them and get a 4-6 stack expunge of against any player worth their weight in lane phase.
But if you do decide to buy him, just always go with a leona and you'd pubstomp most games