It depends on whether your comparison between the two pictures work to justify Timon's different appearance (despite the similar gestures) since there is a different amount of sunlight in both pictures. The picture on the left is clearly morning with Timon with his body facing the direction of the sun hence his shadow while the picture on the right is Timon still in the direction of the sun supposedly in the afternoon, though his shadow possibly on the bottom left of the picture is not quite as thick. For the most of the movie, Timon's appearance looks about the same as he did in the first movie.
However, different animators and two separate animation software programs are the likely culprits for any difference between how Timon looked in the first movie and the third movie. The first movie had the
Computer
Animation
Production
System (CAPS) to digitally color the characters while
1½ had the Toon Boom Harmony animation software program to serve the same purpose.
Anyway, I felt Timon looked more different in
Simba's Pride than he did in the first movie. His fur looks noticeably more darker while his fur in the first movie looked more peachy. Both images below are comparable with Timon displaying a "I got an idea" expression after coming up with an idea originally thought by Pumbaa.