An important caveat is that we haven't had (or at least, I haven't seen) any Texas-specific polling on Cruz since before his pseudo-filibuster, etc, so it's impossible to say with any precision whether Cruz's adventures over the past few weeks have taken a toll on his approval ratings back home. However, as Fernandez notes, people in the state (any state) usually rate their senators more highly than national observers do.
Then, too, there's the fact that Texans are more familiar with Cruz than Americans as a group are.
Relatedly, I'm not sure that Cruz registers as extreme in Texas. Take a look at the election results for the 2012 general:
In Texas, by contrast, the discrepancy between the presidential and Senate contests appears to derive from the fact that the libertarian candidate got about 2% support in the Senate contest, and only 1% in the presidential (and amusingly, the libertarians in that extra 1% seem to have split their votes about equally between the Democrat and the Republican when looking at the top of the ticket).