Not a Canadian lawyer, so I stand to be corrected by someone who is. But in common law jurisdictions generally, there is no such thing as a "valid" or "invalid" signature for simple contracts. Certain types of contract like deeds etc do have formalities that need to be observed for them to be validly executed. But in general, a basic contract requires (a) an offer, (b) an acceptance, (c) consideration and (d) an intention to create legal relations. Whether these things are present is a question of fact, not of law. In theory you can communicate your acceptance of an offer by a text message that just says "k", or a handshake, or a nod, or a grunt. There is no reason why you couldn't do so by sending a thumbs up emoji.

Of course, doing business this way introduces evidentiary difficulties that may make your (or your counterparty's) life hard later on. A lawyer would certainly never recommend you document a contract this way. But a court will nevertheless look at the evidence before it and do its best to determine whether the elements of a contract referred to above are present.