Got this used bike for casual use and light trails, didn’t know it was from Walmart until afterwards (new to all this). Is it trashy or acceptable as a starter?

As a starter and for your intended use I think it’s fine in my opinion. Most bikes sold at big box stores are built with low end components and spec to keep it affordable to most but they’re still half decent and not really bad as a beater. My son had a 26″ Hyper as his first MTB and beat it to heck until he outgrew it. The thing is a good 10 years old but still runs and rides fine.