Do steroid creams make skin thinner?

Potentially. Long term use of topical steroids can cause atrophy or thinning of the skin - and also dilated vessels and possibly scarring. You need to speak to your prescribing physician to determine your exact course which is usually limited to 1-3 weeks depending on your exact underlying condition.
Yes. All corticosteroid creams when used for a prolonged duration (many months) can have the potential of thinning the skin, especially areas with thin skin such as the face, groin, and underarm. But used as directed by your physician, topical steroid cream should not cause skin thinning. Therefore, you should not use your medicated cream longer than the length of time your doctor prescribed.