Mystique is a mutant and a member of the x-men a group of mutant supervillains turned superheroes. Loki is the god of mischief, son of Odin, and brother to Thor. Considering both their powers Loki is much more powerful proving that he is the better shapshifter since he is a sorcerer and has both superhuman strength with superhuman durability. Both have a similar ability in which they both can shapeshift. Mystique has the ability to shapeshift into humanoid objects and could even alter race, age, height, even gender. Loki’s abilities however are based on magic, and he could alter his appearance on a much larger scale compared to her.
Mystique’s shapeshifting is more of physical compared to Loki. Raven has mutant abilities that allow cells to redistribute themselves into a different arrangement while Loki’s abilities are based on magic and more of an illusion rather than the actual change of physical structure. The god of mischief has although managed to turn into inanimate objects that include missiles and has even turned into a bird once. Raven can become invisible which is well out of Loki’s reach. You have seen mystique turn into a dwarf within a few seconds showing the full extent of her abilities. In the comics, Raven even turned into the hulk a feat that requires an incredible amount of strength and concentration for his size.
Additionally, mystique’s shapeshifting is not limited to purely organic compounds, has been seen to assume different clothing proving she can assume inorganic compounds. Raven’s powers have really never been tested to their full extent because she has never had to shapeshift into avery large form. In the comics, she is believed to even be able to assume beings the size of Galactus. This would still not change the mass as it cannot be exceeded. Loki can change his size dramatically. At one point he turned into the midguard serpent, a large mythical snake set to kill Thor eight times the size of the earth.
Raven’s powers are in some ways limited as she cannot possess the abilities of the characters she morphs into while it is obvious that Loki can as he did in the movies. Mystique’s talents are not only restricted to humans she can turn into animals. She can even grow extra limbs or heads to facilitate better combat. Due to the ability to shapeshift Raven has an advanced healing factor that could rival that of wolverine. The shapeshifting also prevents aging explaining that mystique is over 100 years old. The shapeshifting is so good that Loki has managed to fool cities. His illusions probably make you see what he wants you to see.
Ordinary shapeshifting might fool humans but certainly not gods such as Odin and Thor. Loki can also literally mimic the powers of the creatures he morphs into. Mystique could turn a part of a metallic suit into an effective weapon. Both Loki and Raven can mimic the voices of the characters they impersonate but Loki’s is quite more advanced. Based on the facts, it is evident who the better shapeshifter is.