No. Yes, mustard seeds are small, but that isn't their only important property. Jesus doesn't say "If you had faith like a tiny grain of sand", or "The kingdom of heaven is like something really small". He uses the image of a tiny speck of dust, a "mote" when describing something really tiny in another passage, so it isn't like the mustard seed is the only tiny image he's got on hand. By choosing a mustard seed, he is picking something that is insignificant in the wrong environment and immensely powerful, even disruptive, in the right one.
In the parable of the mustard seed, Jesus compares the Kingdom of Heaven to a mustard seed planted in a field or garden. The size of the seed is a part of this metaphor—no one is denying that—but the fact that it is a seed is the bulk of the metaphors power. If the kingdom of heaven were not like a seed or some other transformative thing, it would stay small!
But there's also the characteristics of the mustard plant to take into account.
From wikipedia (citing a more reliable source)
There is a "subversive and scandalous"[6] element to this parable, in that the fast-growing nature of the mustard plant makes it a "malignant weed"[6] with "dangerous takeover properties".[6]
It makes no sense to interpret this as being about the size of the seed but ignore the other "subversive and scandalous" properties. Is it good to have a giant mustard bush in your field? Is it good to have birds sheltering in its branches? What about the other plants in the field—birds are known for eating crops and shade isn't always a good thing.
If you read Acts, you read about people giving away all of their goods and abandoning their homes to partake of the Kingdom of Heaven and about how this totally messed with the power structure of the time and made them really upset—like they would be if a mustard seed got in the garden.
That's spicy faith, not bland faith, and robust faith, not weak.
If we ignore all the other properties of a mustard seed and the mustard plant, and say that this parable is only about size, we're reducing—without reason—the levels of metaphor that a passage from the Bible contains and reducing the power of a metaphor to one that is comfortable and easily accepted.
Looking at the concept, again, of the apostles asking Jesus to increase their faith and his reply, if we focus on size alone Jesus is saying: If you had even a little tiny bit of faith, regardless of it's character or properties, you could move mountains.
But again, Jesus doesn't choose something that isn't a seed to illustrate his point. He doesn't use a grain of sand, although a grain of sand is also small. So why choose a mustard seed? What properties does a mustard seed have that a grain of sand does not?
To focus only the size of the mustard seed and ignore its other properties is to assume that Jesus was speaking in shallow metaphors to people who didn't think very deeply about gardening, seeds, or mustard. That's not historically accurate or particularly complimentary to Jesus. It reduces a potent image rich in meaning to an obvious image with little to recommend it.
