Was it okay to say out loud the views that the Treaty of Versailles terms were humiliating and were to be resented during the post-war Weimar Republic time?
I would assume so in right-wing circles, but what about left-wing mindset? Was it seen as something taboo to say, were there "social justice warrior" equivalents during that time, who shamed you/called you an extremist for saying that the Paris treaty should be revised?
Yes, actually, it's fair to say the they were almost universally reviled.
Starting from the very day the new German Reichstag was presented with terms in 1919, there were attempts to negotiate and change the treaty. The war guilt clause was seen as especially odious, and in fact the Reichstag, under Socialist Minister Philip Scheideman, attempted to surreptitiously remove the language relating to guilt. As this was a recently elected body representing everyone from Bolsheviks to Socialists to Liberals to Conservatives, it's fair to say that this part of the treaty was universally reviled.
Even under the policy of fulfillment pursued first by foreign minister Walther Rathenau and continued by Gustav Stresemann, who would serve as Prime Minister during the Ruhr crisis during 1923 and then as Foreign Minister throughout the 20's, the point was not simply to follow the Treaty but to gain breathing room for German policy makers to slowly revise the treaty's terms. If Germans just demonstrated good faith for a few years, they reasoned, it was likely they could begin unraveling the Treaty's worst clauses, especially the occupation of German lands and the limitations on the country's military. These leaders, and basically all of the Prime Ministers of the 20's, including the socialists, and the Socialist President until 1925 Frederich Ebert, allowed the Reichswehr to pursue clandestine rearmament.
The Bolsheviks were hardly the "social justice warriors" you have in mind. Their brand of internationalism simply held little appeal to the German mittelstand, and they pivoted towards nationalism during the 1923 Ruhr Crisis with, for instance, fiery speeches in praise of the "martyr" Leo Schlegeter.