We have several "testers" who are involved in manual testing and test
automation, but all of that part-time. Some of them are not really
interested in testing and test automation as a field and lose interest
in test automation and improving testing skills pretty quickly.
How should we deal with them?
That depends on why you have uninterested testers.
If this situation exists because you are bad at hiring people, then you need to get better at attracting and hiring folks who want to work hard and be professional testers.
If this situation exists because you pay poorly, you might need to pay more to attract better people.
If this situation exists because you treat your "testers" (it's telling that you put the word in quotes) like second-class citizens, then you may need to treat them more professionally.
Knowing how to "deal" with almost anything requires first understanding how you got there.
Is it okay to have a part-time not-motivated tester?
"Okay" is always a business decision.
I've seen some companies who decide that they don't really care enough to pay and hire real, professional QAers. For them it appears that part-time, uninterested testers check the checkbox well enough.
Other companies value testers and do things differently.
Is there anything we can do to raise their interest in testing and
Again it depends.
The answers ranges from "No, there's nothing you can do", to "Fire them all and hire full-time professionals" with everything in between.