Reasons why it it more likely that Mihoshi is playing dumb
She got to Tenchi's grandfather before Ryoko.
On the Phone with Ryoko
  She said that married stuff, becasue?
   She doesn't consider her supervisor mean and nasty, but
   Ryoko said she was her mean and nasty supervisor.
Peeling potatoes.

She noticed Sasami's reflection.

She seems to pick and choose when to get involved in Ayeka's and Ryoko's fights
Her report.

She found the stolen Galaxy Police ship.
She reported to Washu what happened after delivering the prisoner. (Not on Cartoon Network)

Universe
She navigates her suit through Tenchi's house without harming anything
and then is out of control when using Kiyone's suit? Doesn't that seem to
be on purpose?

And remember "Simulated disorder postulates perfect dicipline, simulated fear
postulates courage, and simulated weakness postulates strength." - Sun Tzu

If you have more reasons, e-mail me at hackwrench@hotmail.com