Rosewall said he wished more sets were played. I think that is quite reasonable (as it is not so much personal). But, then he followed it up by saying DT put up a disappointing show (note, I am just saying from memory and so these may not be the exact wordings).
I have two issues here. First of all, even assuming that he had actually put up a disappointing show, there is no need to say he sucks so explicitly and in front of both himself and a large crowd (especially, given that in that case everybody would already be aware that he put up a disappointing show).
I actually think Thiem fought as well as he could have fought today (except for two issues. #1 is that the initial jitter caused him to lose a game and #2 he had a few botched volleys coming from poor technique). Rafa was not exactly playing great in the previous matches. Starting from Bolleli to JMDP almost everybody had chances to break him often. But, Rafa knew that Thiem is strong and a good clay player. So, Rafa brought his full concentration and power and showed no mercy. Thiem hung in with him on equal footing in many points and actually hit more winners than Rafa (although this almost always happens to Rafa). Thiem quickly regrouped from 0-2 to 2-2 and then played quite decent until the last game. Thiem needed to win the first set to have even an iota of a chance of winning the match and he knew it and all of us know it. So, it is actually quite natural that he got dejected and Rafa sensed and pounced on that opportunity to get an early break and the second set became history. However, in the third set, after going down 0-40, Thiem valiantly fought and avoided the break in the opening game. Finally, in the end he saved about four or five match points. This is not like Dinara Safina or Sabine Lisiki sucking up the joint in a final match totally disappointing the crowd. This is more of an alpha male establishing his superiority over Thiem and not about Thiem sucking. I am not even a fan of Thiem. So, KR's remark is doubly problematic from my point of view at least.
I am sure Thiem will win a couple of RG titles, assuming he does not become too old by the time Rafa eases his stranglehold on the dirt.