Dictionaries pass judgment all the time.

First they decide what they think is worth entering.

Then, if they like, they apply such labels as "colloquial," "informal," "slang," "nonstandard," "archaic," "obsolete," "regional," "U.S.," "Brit.," "Austral.," and occasionally "substandard."

"Hippopotami" bears no label.

One label rarely seen is "Not in technical use." That covers things like "virus" used to mean "any illness." No professional epidemiological discussion would use "virus" that way, even though millions of people do and would, because technically it is wrong.

"Hippopotami," presumably, is likewise "not in technical use," though it's a stylistic rather than a terminological issue.

OED accepts without comment "Plural unchanged, hippoppotamus, hippopotami."

"An Account of Several Late Voyages and Discoveries to the South and North" (1694) tells of "Hippopotami" at the Cape of Good Hope.

Among other serious users of "hippopotami" was David Livingstone in 1865.

Sounding funny doesn't make it essentially humorous.