For me it comes down to the character, not the mechanics.
Does it make sense for this particular character in this particular game to be able to Viciously Mock someone via telepathy? Is the telepathy an important aspect of the character? Does the player play the PC such that the telepathy is used intrusively, obnoxiously against other people? If that is the case, then yeah, I'd probably allow the PC to gain the mechanical benefits that come with that spell. Because it makes sense for this specific character to have developed this ability.
For other PCs? Ones for whom telepathy is barely used, inconsequential, or its use is treated very politely? Most likely, the player themself probably wouldn't even ask to try.
I am very much a story-first DM, and that any game mechanics are only used to enhance the story. I don't care at all about "consistency" of the game rules between PCs, between NPCs, or between sessions or most especially campaigns. So I'd never have any necessity or reason to worry about coming up with "one rule" for this situation that applies across the board forevermore. To me that's pointless. A Nature Cleric can gain a few extra specialized uses out of their nature spells that the war cleric cannot... even if both PCs are in the same game. Just like a telepathy-strong GOO warlock might use its telepathy differently than just a Evocation wizard who happens to have the Detect Thoughts spell as one of their myriad of abilities. And in each case, I'll see what has happened within the game, whether what the player tries or wants to do is effective and/or cool, and makes sense for the way the character has been played. If it does, then sure, I'll allow it no problem. Especially because I keep my eyes open for every PC and what they might accomplish later on, so its not like equality won't eventually find its way in the game.