Not necessarily. The BATTLESTAR GALACTICA reboot was much better than the original. The new PLANET OF THE APES movies are quite good. The ADDAMS FAMILY movies worked, and the HAWAII 5-0 reboot seems to be thriving. HANNIBAL was critically acclaimed and last for at least three seasons. And can we count BATES MOTEL as a semi-reboot of PSYCHO? (The jury is still out on whether DAMIEN is working.)
Granted, there have been some high-profile failures (Hello, BIONIC WOMAN remake), but plenty of original series fail every year, too. I'm not sure reboots have a particularly higher failure rate.
In fact, I rather made a reference in various remake of series of the 70s and 80s, which As a whole, a disaster, were a disaster.