The context is that the chief has left and her coworkers are starting to leave, so she feels anxious that she's the only one left working overtime. The bubble text here is her jumping to the conclusion that the chief probably thinks she's useless as a result. In this case:
'after all the chief probably thinks I'm useless' or
'in the end, the chief just thinks I'm useless' don't quite fit, if you consider the lines before this one.
Since it's a catchphrase, it should be consistent throughout the chapters, which is why 'I bet' works better as it can adapt to different situations and it creates a cutesy tone for her.
When it comes to translation, I guess, it's less about just being accurate, but more about how to maintain the translation accuracy while paying attention to its suitability in terms of context, characterisation, etc. So yeah, that also gives you flexibility in the translation choices you make. It doesn't have to be 'I bet'.
But then again I'm the translator for this series and I just plug the text into Bing TL while smoking weed anyway so idk what I'm saying.