It is only illegal (in Texas) if it is offensive to someone. Since people are offended by practically nothing these days, then chances are there would be an issue with it. For Public Lewdness, you need an "act"...and for Indecent Exposure you need intent to "arouse or gratify". Just walking around the yard totally butt naked for no apparent reason does not fall under either of these offenses.
This would be covered under local ordinance/municipal code.
Texas Penal code violations:
§ 21.07. PUBLIC LEWDNESS. (a) A person commits an
offense if he knowingly engages in any of the following acts in a
public place or, if not in a public place, he is reckless about
whether another is present who will be offended or alarmed by his:
(1) act of sexual intercourse;
(2) act of deviate sexual intercourse;
(3) act of sexual contact; or
(4) act involving contact between the person's mouth
or genitals and the anus or genitals of an animal or fowl.
(b) An offense under this section is a Class A misdemeanor.
§ 21.08. INDECENT EXPOSURE. (a) A person commits an
offense if he exposes his anus or any part of his genitals with
intent to arouse or gratify the sexual desire of any person, and he
is reckless about whether another is present who will be offended or
alarmed by his act.
(b) An offense under this section is a Class B misdemeanor.
Here is a revealing web page for additional reference...
Nudity and the Law