To answer that you need to look at generations much closer to yourself.
It is OK to have Geni profiles of these mythical persons to visualize their genealogy, even if the sources is "just" sagas.
The question is if there should be any connections back to them.
Nobody can prove European lines behind the middle ages except some royal and noble lines, but no one can be proven so far back as to the biblical, viking and mythical lines.
The keyword for the weak spots in the line is royal and noble connections, a desired connection for most people all the times.
The old kings and other powerful people fabricated them to justify their power, and in more modern times professional genealogists add them to give their customers value for their money. Once published, and even worse if printed in a book such connections spreads like a virus.
If you look at your own line I bet you will find female connections (and other strange connections) to royal or noble lines, some even so visual as being named unknown. They are usually the false connections and you should take a discussion with the profile managers about getting sources and if not, cutting them.
I mentioned female connections because the main problem, is that in those ages the mothers of a child was almost never mentioned in primary sources, so they can neither be proven or dis-proven.
Colleen Dorothy Maunsell the answer to your question is really a bit complicated :)
What does "myth" mean in this context is an important first question. Someone being mythical doesn't necessarily mean they did not exist, only that we don't have good evidence for them :)
The next question would be what is "good evidence" for a person who apparently lived a two thousand years ago. You'll get lots of different opinions.
After that think about "... Geni tells me ..." because this is a little simplistic, Geni hasn't made the connection, people adding profiles to Geni over the last decade plus have created a chain between yourself and Fornjot and Geni is reporting that chain. Without "good evidence" for every link in the chain then the chain itself is ... questionable :)
There are 14 discussions linked to the Discussions tab on Fornjot's profile and most of them are touching on this concept to some extent.
Fornjot "the Ancient Giant", King of Kvenland is your 43rd great grandfather
King Kari ''Wind'' Fornjotsson Fornjotsson, King of Kvenland is your 42nd great grandfather.
https://fi.wikipedia.org/wiki/Kvenland
Kvenland was a large part of Modern Finland and part of Sweden. there are still things in the terrain in finland that lead the quenland head to the city
https://geographic.org/geographic_names/name.php?uni=-1975583&f...
https://en.wikipedia.org/wiki/King_of_Kvenland
I like the fact that these characters are listed as mythical and that they are included in the "story" of us. As long as we are upfront about them being mythical. If we lack the evidence to recognize them as real people lets celebrate their tales as a part of our heritage. Stories need to be retold to be remembered!
The ancient giants, well it's likely not a real GIANT, but a paraphrase expressing a man ruling many people under him, this rewording of something to somethong else is common in the building of myths, often, its just based on real once existing people who have been reshaped in order to express their qualities or signum and make the story powerful and memorable. There is more likely truth behind many of the sagas that have survived into our days, but we have to read between the lines in order to understand the basic meaning, not take it in a literal manner or sense just as it was exactly so and such it happened. Even the name "FornJot", = ancient giant, says that this wasn't a real name, no one, would ever name a child to that... but the person, behind the tale might just as well have existed, but with the difference that his real name actually were forgotten at the time the tale was popularized.
Example of real once existing people who today is know by different name.
Gorm the old. Was never known under his time of being called that, He might have had the name Gouthrum but shortened to Kurm and or Vurm, and he died before the age of 50, not that old at all.
Should we therefore doubt his existence? With lack of evidence, he night as well have been considered as a mythological king and we all know by now, who would have supported that kind of ideas here on Geni in a flash without any reflection.
Another example of an once living person, turning into a myth.
Beowulf, one of the oldest anglosaxon text in old english downwritten around 1000, a copy of an older manuscript from around 600.
https://sv.wikipedia.org/wiki/Beowulf
https://en.wikipedia.org/wiki/Beowulf