Desktop Linux distros (including Ubuntu) don't sign most of their image. Generally, only the bootloader and maybe the kernel are signed. This allows the use of Secure Boot to block "bootkits" that replace pre-boot code to do something malicious (such as start a hypervisor that gives the attacker full control of the running OS without the OS knowing). However, it provides no other security guarantees.
Even on Windows, which signs far more parts of the operating system, signature validity is not generally enforced; aside from critical stuff like the security subsystem and kernel-mode code, you can tamper with system files and the OS won't even notice unless you specifically tell it to check. On Linux, most stuff isn't signed at all, so there's no signatures to check.
Now, with that said, the classic reason for verifying ISOs before flashing doesn't really apply anymore. All ISOs these days should be downloaded over secure (e.g. HTTPS) connections, where an attacker can't tamper with the download unless they can fully impersonate the server. If they can fully impersonate the server, then they can lie to you about what hash digest to expect, so attempting the verify the download integrity is mostly futile anyhow. Furthermore, HTTPS also validates that the traffic hasn't been tampered with accidentally (e.g. bit flips from cosmic rays or whatever). You might want to validate an ISO before writing it if you're worried that your RAM or storage isn't 100% reliable... but in that case, you really should repair your hardware before installing a new OS.
Verifying an image after writing the ISO is potentially still a good idea (though trickier than it was in the days of CDs/DVDs). Flashdrives have limited durability and can introduce errors. But that's still very low risk, and you'd only be worried about accidental errors (unless the computer you used for the flashing is compromised by an attacker, in which case you shouldn't use it for anything much less downloading and installing an OS).
