I disagree with that. TT has proven himself to be an average QB over 2.5 years. If you even want to say slightly below average i'll grant you that, but he doesn't suck.
RG3 was amazing his first year and sucked after that because defences started to catch on to what he was doing. Remember in 2012 the offence that Washington ran with him was almost brand new at the time, and was pretty much a college playbook. D coordinators simply were not prepared for it that first year.
TT if anything got slightly better the 2nd year, and probably stayed the same this year which is a problem because it wasn't good enough. He also stayed the same or even slightly regressed this year because it was a new offensive system that does not fit for him.
TT needs a simple offence tailored towards him for him to be successful. You can say he is limited, but he's not a gimmick QB like RG3