Using a dictionary would be slower than using a simple int array as OP has done. You're adding more overhead/complexity and probably memory usage as well by using a dictionary.

OP's solution is exceedingly fast (see my comment about his error, though). Even much faster than my own (using a short str1 input...really long ones, my code is much faster), which is faster than all of the other solutions posted here that I've tested.

Per kata author's instructions, only lower case letters will be provided. Which means zero non-letters, whitespace, digits, null/empty strings.

I'm nitpicking, but starting for loop from 2 would be give the same output.

+1 Efficiency, only concern is if str2 is empty string then it reports canScramble true, but that's just me being difficult :P