I mean, it’s a fairly major issue of legitimacy coming hot on the heels of a literal civil war. Caesar being salty for a few days is a relative non-reaction.

Though, in favor of the pettier view of the subject, Caesar had been cheated out of the triumph he was given earlier in his career (a triumph being the moment of a Roman politician’s career representing extreme success), and so was likely not in the mood for anyone (metaphorically) raining on his (literal) parade.