It seems to me that there are two different concepts being discussed here; checksums and signatures (or integrity and authenticity).
@bedwardly-down 's initial comment (above) was regarding providing some kind of checksum file as an integrity check so that a user can check the install package against a checksum just to ensure that the file downloaded correctly. All you have is an additional file that can be downloaded with contents something like 2e78e3596d82df90f5b554172495955f6d807ae2 *Joplin-Setup-1.0.201.exe that can be compared against what was downloaded. Nothing about this has any kind of authenticity element. It is an integrity check only (i.e. check the file is complete), a trouble-shooting aid.
However the other thread that @tessus refers to concerns something different which is GPG signing install packages to ensure authenticity.
MD5 and SHA1 are certainly not good for encryption but they are more than adequate as a simple checksum for just helping to confirm a download came through intact. Even today MD5 and SHA1 checksums are used for integrity checks when creating forensic disk images.
If someone is going to come up with a malware laden version of Joplin they are not going to try to also engineer a modified version that somehow matches the MD5/SHA1 of the original. If they can compromise an install package and get it into Joplin GitHub releases it would be easier just to replace the MD5/SHA1 checksum file as well. It doesn’t matter whether you use MD5, SHA1, SHA256, SHA512 or whatever because none of them provide any authenticity.
This is where signatures using something like GPG come in. These provide the same function (to confirm that a file is intact) but they also provide authenticity as private keys are required to actually generate the signatures. To be done properly the full-on air-gapped, multi-part GPG signing route requires some very serious effort.
What option is chosen depends on what you want / need to achieve. If the software is a prime target for compromise then GPG signing (along with proper implementation protocols) is a good option. However if you just want a user to be able to see if their telco’s unshielded three strands of cracked copper wire into their house has flipped a few bits in the download, then a simple MD5/SHA1 checksum will suffice.