Dictionaries don't pass judgement in the sense that they don't tell you what's right or wrong. It is their job to define words, to include all words that are in common usage and to interpret contexts in which words may be used. Of course, the latter requires judgement, but not in the sense you meant.

As for hippopotami, it's plainly not wrong, but as for how its usage is regarded it all depends on where you look it up. For example, from lexico.com:

"Other words ending in -us show a very varied pattern. Like octopi, the plural hippopotami is now generally taken to be either funny or absurdly pedantic, and the usual plural is hippopotamuses."

We can all indulge in confirmation bias.