Yes, it is generally safe if you take a few precautions. Most converters only read your MBOX file and write the emails into a new PST, so your original stays unchanged.
Before converting, copy your MBOX files to a separate folder as a backup. Download any tool only from the developer’s official site, since fake free converters are a common way to spread malware.
Then test with a small file or a free trial first. For example, Kernel MBOX to PST Converter has a trial that handles 25 emails per folder, which is enough for a quick check. Open the PST in Outlook and compare the email count, folders, attachments, dates, and any non-English text. If everything matches, convert the full mailbox, and keep the original MBOX files for a while afterward.