- Regal Leosaur is a "spell with mana value 2 or less", because the amount of mana in its mana cost is 2, even if you pay more than two mana to cast it, but
- Parcelbeast is not a "spell with mana value 2 or less", because the amount of mana in its mana cost is 4, even if you pay two mana or less to cast Parcelbeast (whether as a mutating creature spell or otherwise)
(C.R. 202.3). A spell's mana value is determined regardless of what was spent (or will be spent) to cast the spell (C.R. 202.3). The only thing that can change a spell's mana value is the value of X chosen for the spell, and then only if the spell has in its mana cost (C.R. 202.3e), which neither Parcelbeast nor Regal Leosaur has. See also this thread.
EDIT (Jun. 9, 2021): Edited to conform to rule changes in the meantime.
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