Thanos likely knew of Hela and respected her power as an Asgardian threat, but he wasn't necessarily afraid in a fearful sense; rather, he recognized her immense power, which made conquering or dealing with Asgard complicated, though he ultimately waited for her to fall after Thor defeated her, allowing him to easily take the Tesseract, indicating a strategic approach to dealing with strong opponents. In the comics, Thanos is deeply infatuated with Lady Death (sometimes conflated with Hela), a relationship that drives him, but in the MCU, he's more focused on cosmic balance, yet still wary of powerful forces like Hela.
In the MCU, Thanos was never afraid of Odin. This is a common misunderstanding. People assume Thanos waited until the time of Infinity War to collect the stones because Asgard was destroyed. He actually waited for the time of Infinity War because that's when he captured Nebula, and from her, learned…
Yes you are wrong, Thanos is far more powerful, Hela resides over a realm of death, Thanos defeated actual death herself and death is where he gets his magical abilities from, so his magic and the versatility of it surpasses Hela.
In the comics, Thanos, the Mad Titan, was trying to impress Lady Death, Hela, the literal embodiment of death, by killing as many people as possible. It was all about love, his desperate need for it.
And much like the grim reaper, Mistress Death's true form is a skeleton but she can changed into a woman (if so desires) and if anything happens to her (like being captured) then death itself would stop. Thus, in the comics Thanos actually falls in love with her and wants to prove his love to her.
In this final confrontation, it was only Loki's resurrection of Surtur and the ensuing start of Ragnarök that allowed her to be defeated. Odin is the only one known to have defeated Hela in battle, having cast her out of Asgard and imprisoned her in Hel.
She also releases her spirit as an astral projection, which retains much of her powers. Hela is weakened without her cloak—so much so she cannot keep herself standing or cast magical power. It's theorized that without her cloak, she cannot leave her two realms.
Thanos is an omega level mutant.
She didn't say it was “weak”… she said it was “fake”. And the reason she said that was because of a plot hole in Thor (2011).
Superman, Martian Manhunter, Darkseid, Icon, Spectre, Batmite and Myxzptlk, Highfather, Orion and all of the other New Gods etc. Superman, Fate, Lantern even batman can defeat thanos. Remember infinity stone is not thanos's power.
However, Red Skull saw his true intentions, informing Gamora that Thanos was, in fact, weeping for Gamora, as he truly loved her as a daughter, and was heartbroken that he had to sacrifice her to get the Soul Stone. Disbelieving, Gamora could not comprehend that Thanos ever showed genuine love for her.
Thanos also displays many warning signs of a psychopath throughout the Avengers saga.
Losing Himself
Not to go too far down the rabbit hole of pop psychology, but what Hulk and Banner are most afraid of is losing themselves to one another.
Omega level is a world level threat that's why. Logan can't effect the entire world with his skill set or powers. Where as storm can change the weather across the entire planet.
Who is Thanos? Thanos is a genocidal warlord from Titan who has a lifelong objective to bring stability to the universe by wiping out half of all life at every level. To accomplish this, he collected the Infinity Stones, and he used the Infinity Gauntlet to harness their power.
Unfortunately, now that we have a multiverse, the MCU is anything but "tight and concise." This, I think, has been the main problem; the multiverse was the bad answer to the question "Where do we go after Endgame?"
A space on all things Thor. Why did Hela call the Casket of Ancient Winters weak when it can freeze an entire planet? Because that isn't very impressive compared to her. Or the Eternal Flame.
Overall, while a fight between these two would be incredibly close, Scarlet Witch's reality-warping abilities give her a significant advantage, making her the more likely victor. Hela could win this if she's dealing with an inexperienced and perhaps overly emotional Wanda.
Hela has vast mystical powers which she can use for various effects like limitless astral projection while retaining many of her powers and abilities, firing deadly bolts of energy from her hands which can age or even kill Asgardians, levitation and the creation of illusions, she can manipulate the dark magic.
Unlike other Asgardians, Hela was immortal – not aging while imprisoned for 5000 years. In addition to her accelerated healing factor, Hela drew increased strength and power from Asgard itself as the first born of Odin.
Mjonir is still incredibly powerful… but it's no longer an invincible weapon. Thor doesn't even have time to dwell on the impacts of his father's death before Hela shows up, destroys his beloved hammer, and bifrosts boots him to a junkyard planet. Hela easily grabbed the weapon and crushed it.
Kamala became obsessed with them, especially Carol Danvers, AKA Captain Marvel. She modeled her ideals after her heroic idol, and when she gained unexplained powers herself, she learned to use her powers for good.