Well, you've got good points, but that's the worse things to look at.
Here's what's different and good about pichu.
+Better Aerial mobility (significantly vertically, and enough horizontally)
+Better recovery distance (like meleebrawler said)
+Fair and bair have more knockback (so, better for gimping, including thanks to recovery and mobility)
+F-tilt is not only a move that can trip at lower percent, but is a ko move around 110-130%, which is great for its speed.
+F-Smash has more power (but -less range).
+Aerials have less landing lag and better frame data in some cases. (faster attacking can mean more pressure, and ability to get out of the way after)
+Up-air has some potential combos into thunder. (fall speed being better helps gauge cloud distance more, etc)
+D-tilt has potential for combos as well. (like you said)
+Side-B is faster and may have more usage than pikachu's (including maybe even in some combos)
+Neutral-B (t-jolt) may be a little bit better (will have to check more)
And that's about all I know up to now. Let's see what comes up once the game comes out.
But yes pikachu may be objectively a better character, but that doesn't mean pichu isn't a good choice for some. And if it's thought to be 'bad', that's all the more reason to play it, and catch people off-guard
It's also an 'underdog' choice.
Also..
+Cuter. <3