I just joined an aviation company but the learning opportunity is rather limited due to COVID-19. The slowdown is expected to last awhile. Should I start to look for a change in job?

Indeed the slowdown for the aviation industry may last for a while.
Perhaps you can look at what made you join the industry in the first place.
Was that part of a longer term plan?
If you choose to stay because it is an industry you want to stay in, how can you still find opportunities to learn be it inside or outside of the company?
If you choose to change, don't rush into it. Do compare the pros and cons as the current market situation is quite uncertain.