Not really. Thiem is just a choker. His tennis IQ is not as good as the Big 3 and he is not mentally tough at all. Compared to guys such as Murray, Wawrinka, Delpo etc even Cillic.
I find your username ironic - are you really a fan of tennis? To call a player in the Top 10 players in the world - and the second-best player in the world on clay - the toughest surface - "just a choker and not mentally tough at all" is ludicrous.
Sure, he may not (yet) be comparably as tough as some others, but then again, some of those weren't as tough as they are today at a similar career stage. Djokovic used to rely on injury breaks and retire if he was losing, Federer used to smash racquets, even Nadal who has always been pretty mentally tough has much more confidence to go to a plan B than he had previously. Funny how much confidence can be drawn from a first slam win, etc...