Leaving aside whether we agree with the principle of strikes and how difficult / easy they should be to call, we can see a few issues here. For a start, agency workers wouldn't make a bit of difference to today's strike; you can't just ship in a bunch of temps to run a massively oversubscribed transport system. Also, Johnson might want to look at the turnout in the 2008 Mayoral elections (45.3%) before he starts trying to confer illegitimacy on thresholds under 50%.
But the other, more serious, issue is the question of the much vaunted "no strike deal" bandied about by Johnson during his election campaign. Last month, Dave Hill reported on the titanic struggle of Martin Deutsch to get Freedom of Information answers from TfL about progress on this deal. TfL eventually, after a written answer from the Mayor saying "TfL has had discussions with each of the four transport trade unions", declared that it doesn't hold any information on what happened, because the discussions were "private conversations" rather than "formal meetings".
For all we know, these private conversations could have gone along the lines of :
TfL: "So, how about this no strike deal, then?"
Union: "Piss off, mate."
For more than two years in office, a couple of quick chats on the side doesn't seem like progress. Perhaps the Mayor's office might be better served in delivering the promised negotiations, however distasteful they might find it, while the rest of us get wet walking to work.