Sure, you said that can use it to mean 'now you'll want to surrender because you want to join', okay the sentence can work in that context, somewhat; but know this: Because /ff means giving up the match, so that line "type /ff to join Islam" CAN also carry the message that only losers join Islam; and as mentioned by another commenter, it can also be a malicious insults to associate bombs to Islam considering your nexus explodes when you pass the /ff vote. Since it goes both ways, it can either just be a random comment or it can be a serious insult, let's just say the opinion can be pretty mixed when receiving it especially from strangers, and especially when the comment seems to carry more negative implications than it does positively.
Really though, wanna do ambiguous jokes? Cool. But at least either be very witty about it, like very, so that the intention (that is not to harm) is clear, or you might want to restrict it to someone who knows you or else the audience may not be taking it well if it's not really good and clearly well-meant and/or satirical.
Not to mention, at the time of political tension like what we have regarding the topics on Islam, it's also just not very smart in general to whip out ambiguous jokes about the subject to strangers out of the blue for many obvious reasons.