I have watched Young Sheldon up to the latest episode. Whenever the theme song is played, Sheldon is shown moving away from the cow. Why is it like that?
It’s a parody of the “Walker, Texas Ranger” (1993) intro. This answer is found on IMDb trivia, and is pretty apparent when you watch the two. It’s just a trope: Texas cowboy hero looks off into the distance in the midst of the majestic American countryside, awestruck and pensive..
The addition of the cow is just a joke like others said…disrupting the “epic” moment and Sheldon’s fantasy of himself.
As the show progresses I think they just add the other family members and change the outfits as a creative choice due to the show’s success. From IMDb:
Beginning with season 3, the opening switched from just having Sheldon walking to his spot to him walking up to the rest of his family, but dressed up differently in every episode but the first, leaving his other family members initially looking at him puzzled.
I think it is just to throw Sheldon off. He is well known to be germophobic, and have some pretty irrational fears...so the cow ruining his perfect shot and then even stepping toward him is to give the audience a smile as he uncomfortabley steps back to put slightly more distance between him and the cow
I am guessing you are talking about the end of this sequence:
At the end the family is obviously posing in a similar fashion to a monument of great american heroes. The background also would be quite suitable to the western genre. So two soft connections to the founding days of a almost constantly great nation.
The protagonists look up, slightly to the side and are motionless like statues. They are disconnected from the viewer, clearly aspiring to something great.
Then they all turn their head, which is quite uncommon for statues, let alone monuments. So they already get their first crack.
Then the camera pans and shows an ordinary cow standing right next to them. Ruining the perfect wild-west-monumental-panorama.
Then the cow even dares to take a step towards them.
The mighty, aspiring statues back off, but only a tiny, marginal bit.
Then they resume their monumental positions again. Once more magnificent, just with a cow next to them.
Addition: Some of my example statues are of the dark chapters of American history. I am not sure if there is really a pattern there. But possibly it could also be a common trait of redneck-ish statues to be disconnected from the common folk in front of them.