Can I shorten this assembly code to test if a number is even?
As was helpfully pointed out by G. Sliepen in a comment, you can flip the bottom bit using xor.
That enables the above code to be rewritten in one fewer instruction:
isEven:
  andi $t0, $a0, 1
  xori $v0, $t0, 1
  jr   $ra

This takes advantages of the properties:
A xor 1 = !A
A xor 0 = A