Although it is not unique to Marvel Comics, which means that Stan Lee's own poor memory is somewhat irrelevant, I would imagine that it did begin as a mnemonic device to facilitate easy recollection of the characters' names, which became a longstanding tradition in the comic book industry. In my opinion, it's just too common to be explained any other way:
Believe it or not, this is nowhere near a complete list - for example, Vicki Vale and Beast Boy are missing, as are many, many more in the DC and Marvel universes. Other comic book publishers are no exception to the rule - e.g., Archie Andrews and Jughead Jones. Cartoon characters with alliterative names include Mickey and Minnie Mouse and Donald Duck, Fred Flintstone, Bugs Bunny, Daffy Duck, and Mr. Magoo. And long before any of these, we had Peter Pan.