Shella is like a high mid tier in my opinion. Shella is slow when moving and attacking, but has a very fast dash with endlag. But you can cancel the dash by crounching, jumping and with the up attack. That might not seem crazy, but trust, it is.
Thanks to the fast dash speed, you can zoom in and out really fast by wavedashing. By dashing canceling with the up attack, you can get into your opp already attacking from a mid distance.
And last but not least, by dash jumping you inherit the dash speed into the jump horizontal speed, letting you punish fireballs and even combo extend after Shella's dash attack.
Sorry if it was a little confusing, but i really think Shella is good, just takes some pratice.