Is it appropiate to put a funny funny slide in a Bachelor thesis presentation? I always like to stray in a context related funny slide either at the beginning or at the end of a presentation because I do not like the overall stern atmosphere at presentations.

I also planned a funny slide that is related to the topic of thesis at the end of my Bachelor thesis presentation but I am not sure If I bring myself in jeopardy through this because I assume they have to be strictly just presenting the mere content of my work and nothing else. I am even afraid that my open question to the audience (which includes Professors and Students too) at the beginning of my presentation is regarded as a violation of this strict code.
I'd consider this a classic example of ‘if you have to ask, the answer's no’.

To expand a little on this: I don't think that humour is always forbidden in academic contexts. I've seen it used in both publications and presentations -- only very rarely, but usually successfully. The problem is that ‘appropriateness’ is massively context-dependent. It depends on exactly what kind of humorous content you have planned, exactly how you present it, the culture at your institution and department, who's in the room, what mood they're in, how your audience regard you, what preceded your presentation, what you have to lose if your attempted humour fails, and a hundred other factors. You are far better placed to assess all these factors than anyone else in the world -- especially strangers on the Internet who know absolutely nothing about the details of your situation. So, if you yourself are unsure whether it's appropriate, there's probably nobody here who can give you a more reliable answer -- which means that you should play it safe and leave the humour out.

One thing you might consider is addressing your question to colleagues in your own department, who will be far more familiar with the particular context of your presentation.