I think Tiergarten Wels is a great place for what it is: a small park with some animals and free admission.
It feels more like taking a walk through a public park, but with some interesting animals along the way. That makes it especially enjoyable for families with young children.
Of course, you should not expect a zoo. Some enclosures are quite small, and personally, I don’t think that animals like a lynx belongs in such a limited space. However, for many of the other animals, the park offers a pleasant opportunity to see them up close without spending any money.
If you are already visiting the Messe Wels, Welios or the Volksgarten, it is worth a visit. Just think of it as a relaxing walk through a park with a few exciting animals rather than as a zoo experience. If you are looking for larger and more complete wildlife parks in the region, I would recommend Cumberland Wildlife Park in Grünau, Zoo Schmiding, Tierpark Haag.