When you're mistrustful, you have a sneaking suspicion that you should be wary of someone or something. If you're mistrustful of clowns, you keep your distance from them at birthday parties and circuses.
If someone is mistrustful, they have a general lack of trust, or a suspicion. You might be mistrustful of things you read online, or mistrustful of teachers who assure you an assignment will be fun, or mistrustful of unfamiliar new foods. This adjective is very closely related to distrustful, although being distrustful is typically based on experience, while being mistrustful is more likely caused by a general sense that something's not trustworthy.