You can combine three lesser mutagens into a normal one and three normal ones into a greater one.
3 Lesser Red Mutagens > 1 Red Mutagen
3 Red Mutagens > 1 Greater Red Mutagen
The majority of mutagens you have though are probably monsters'. Wraiths must have a near 100% chance of dropping them since my inventory is filled with those.
Monster mutagens are only useful for decoctions though since they offer the same bonus as the lesser mutagens and can't be combined. My suggestion is to just sell them, or drop them somewhere, they aren't worth that much money really.
