[Dark Sun] Are Thri-kreen overpowered?

It's not just you.

At low levels, it's a fair summation.

It ends up balancing out -

Only race that has less class options is half-giant.
Can't use armor or other stuff that requires human/oid form.
RP issues with other races due to their tendency to eat them, elves in particular.
The really good racial abilities don't kick in until 5th-7th level.

That being said, until the other races level more, a Thri-Kreen will out-do them in combat a lot of the time with ease, and can do it without weapons.
If you want to really clean-up, any combination of Fighter, Druid, Psionicist is the way to go.

There is one thing however that really pulls things back into balance (yeah, I know some people hate that term).
Thri-Kreen age.

They start at age 6, venerable at age 25, maximum age of 25 + 1d10 years.

One good whacking by a creature with an aging effect, magical spell, item effect, curse, etc - and you're done.
Aged to death.
If you survive a campaign and reach high levels (or even mid-levels), most people will still be happily playing their more normally aging alts, but again - you're done.

My brother and I used to play the two Dark Sun PC games back in the day before I ever got the boxed set.
You never made a party without a Thri-Kreen.
They were what kept you alive while in the gladiator pits.

In the Pen and Paper game though, it's a maximum punch at low level, everyone gradually catches up, then you die off really quickly due to above-mentioned issues.

Takes me fucking ages to say anything....

TL;DR version: There are a number of down sides to playing them.

EDIT: There's a section on Thri-Kreen vulnerabilities on page: 28 of the book "Thri-Kreen of Athas".  Far too many to list, but in general most of the advantages of the race also have a drawback. It's up to the DM though to remember to keep it in mind and add them to various situations.