Very possible, yeah. Though there are 100% blind spots in the rules if I'm being honest. The one I always come back to that made me go "...Huh. Never thought of that" is when Don'Tale Mayes took Josh Parisian down, got on top of him, and put him in north-south before delivering a series of... pretty vigorous pelvic thrusts to his head. Josh (understandably) freaked the fuck out on bottom and looked to the ref, who was equal parts confused and dismayed and told Mayes to stop without really giving a reason as to why. It was super awkward, but I'm not sure if this is actually against the Rules and if so under what grounds. I mean it's not a kick or a knee to a grounded opponent, but it's arguably worse lmao
And it's possible that Mayes was legitimately causing real damage (and not just the emotional kind) with those pelvic thrusts, too. A lot of fighters wear steel cups into the cage. When you're talking about a big, 250+ pound guy like Mayes putting a significant portion of his body weight and core strength behind each thrust of said cup and jackhammering downward against a dude whose head is pinned between him the mat... not pleasant to think about.
Nothing explicitly bans tickling, though if a ref really wanted to get onto you about it this is the closest rule I could find:
The spirit of this rule is clearly to keep someone from raking at their opponent's flesh with their nails or giving them the ole Indian sunburn or something (which is a hilarious mental image unto itself, mind you). But I guess if a ref wanted to interpret it in such a way as to discourage aggressive or painful tickling they maybe could. Not to mention the blanket bans on unsportsmanlike conduct and not following a ref's instructions.