It is true. La Liga competition is simply more competitive than Bundesliga. There are more strong teams in La Liga, Lewandowski can't easily score as in Bundesliga. Age, teammates, and tactics also have big impacts on Lewandowski's performance. These are basic factors that directly bring serious effects on Lewandowski. So, we can't compare his goals during in Bundesliga and in La Liga.
There are so many strong teams in la liga and bundesliga. There's no need to compare any league caused by they have their own characteristic. I think that if you were not even watching it correctly.
It's better for you to read this article.
https://www.bundesliga.com/en/bundesliga/news/robert-lewandowski-s-record-collection-bayern-munich-7764Lewandowski has been making so many record in bundesliga. Think about that. It seems like that you rarely watching bundesliga, right? Lewandowski's performance is not having a lot of differences when he came to the la liga.
He was able become the top scorer in la liga on his first season with barcelona. that's enough to prove if lewandowski is still a good striker while he gets even older from time to the time.
Benzema is almost surpassing lewandowski as top scorer in la liga right now. Benzem has less goals caused by he was facing injury during the early season.