There are a couple of things, but I want to emphasize that I don't think any of these should change... I like it as it is.
The Mockingbirds main gun is AP -, which makes it always make a Glancing hit with -3, so it can basically never destroy a vehicle. Unless it's target has taken a couple of hits before.
The lack of long range anti vehicle weapons. 5th is all about troops in Rhinos
Expensive units. The cheapest infantry unit cost 12ps and is basically only good for taking objectives and hiding in cover. Expensive transports.
You usually only have one Shadowseer to protect your Harlequins in your army if you want to benefit from any of the Great Harlequins upgrades. Maybe the Shadowseer should have been an Elite and the Solitair HQ?
So-so HQs if you compare to Eldar and Dark Eldar, low stats and few upgrades.
I agree that the IC don't do much and my Harlequins too do all the damage. After just 2 test games I have now added a 4th small Harlequin unit (that will Deep Strike).
15p is way to much for a Mime, compare it to a Trueborn, Wyche, Striking Scorpion or a Space Marine... I think they probably should cost around 10ps and have a Plasma Grenade upgrade option.
But again. I like the synergy of the list and am having a lot of fun with it. Maybe some more tweaks for 6th ed next year and it will be perfect