If you've certified only the real keys, the real Electrum will show that it has "valid signatures" instead of what you've seen "the data could not be verified".
I haven't mentioned but, before certifying a certificate, it's very important to look for some legitimate source where you can verify the fingerprint.
For example: ThomasV's certificate that I've imported has a fingerprint 6694 D8DE 7BE8 EE56 31BE D950 2BD5 824B 7F94 70E6 which is
the same as the one in the official documentary: https://electrum.readthedocs.io/en/latest/gpg-check.html and from various users' replies.
But better if you can verify it from the person himself: https://github.com/ecdsa