Should you show that you use AI in your portfolio?
Yes, if you explain what you did yourself: the idea, the choices, the retouches. What matters to a recruiter is your eye, not the tool.
Which AI tools should a graphic designer learn?
Start with those your usual software already offers, like generative fill, then try an image generator on a real direction search. What matters is fitting it into your method.
Can you use an AI-generated image for a client?
It depends on the tool: read its terms on commercial use, and tell your clients. Avoid asking for an artist's style or a specific work, and keep a record of what you retouched yourself.