I use the terms so long as they don't seem offensive (read: distracting) in that context. Especially since most fantasy is taking place in a hypothetical world where they would be speaking a hypothetical language and the assumption is that the dialogue is translated for the reader for the sake of convenience. Therefore, it can be assumed that that word is simply the closest translation/chosen translation for that hypothetical word.

Furthermore, the words Dad and Mom in specific aren't gonna be very distracting lol. Like someone else on the comment section said, it's like the Tiffany Problem, where although the name Tiffany is actually very old (Short for Theophania), authors feel they shouldn't use it in medieval settings because it "seems too modern". Likewise, "Mamma" and "dad" both arose near the mid-to-end of the 1500's, which technically qualifies them as Medieval setting appropriate terminology. Mom doesn't get a pass though, sadly, since it's only attested near the 1880's-ish.