No, Jesus is not 'lessening' the dietery law ('can but shouldn't'). He is explaining that it does not pertain to the New Covenant ('what you eat cannot be a sin intrinsically').
The Law had several aspects, such as the ceremonial and dietary laws particular to the Jews as a nation (for the purpose of creating a nation humble under the rule of God, typifying the 'grafted in' new "Israel of God," the church 1), not to the New Covenant people which would necessarily adopt the moral-law aspect of the Law (necessarily, since they are intrinsic moral laws, such as "thou shalt not murder," which is universal, not something extrinsically forbidden by God for some purpose other than that it is intrinsically sinful—such as sanctification: a kind of penance).
1 Gal 6:16; Rom 9:6; 11:17
An Important Distinction
Just as St. Paul makes the distinction and clarification that it is not the outward working out of the evil or good that makes a man accountable for evil or for righteousness, but the inward disposition and intention to do such (if it is something that can be 'done' visibly), Our Lord makes the distinction between the outward appearance or result of an evil thought, and the evil within the heart from which it sprang:
A good man out of the good treasure of his heart bringeth forth that
which is good: and an evil man out of the evil treasure bringeth forth
that which is evil. For out of the abundance of the heart the mouth
And calling again the multitude unto him, he said to them: Hear ye me all, and understand. There is nothing from without a man that entering into him, can defile him. But the things which come from a man, those are they that defile a man. If any man have ears to hear, let him hear. And when he was come into the house from the multitude, his disciples asked him the parable. And he saith to them: So are you also without knowledge? understand you not that every thing from without, entering into a man cannot defile him: Because it entereth not into his heart, but goeth into the belly, and goeth out into the privy, purging all meats? But he said that the things which come out from a man, **they defile a man. For from within out of the heart of men proceed evil thoughts, adulteries, fornications, murders, Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within, and defile a man.
A very important truth is taught here, not just about food—even more important than St. Paul's distinction about grace and works (void of or insolation from grace)! And that is that evil is accounted as evil (and good accounted as good) based on the intention of the heart, and not the playing out in space and time of that intention (the work of good or evil).
Consider a Scriptural analogy:
Abraham was considered righteous before he did the act God commanded him to do: on account of his faith that God could raise even the dead, he in an equivalent manner to having recieved back his son from having died, he recieves his son's life from his expected, and from Abraham's perspective, sure death. 3 In this, his intention was to do the will of God, and so as far as God and Abraham were concerned, Abraham was as good as having already fulfilled his intention currently, and so could presently be considered righteous—all without his having physically brought about his intention! This proves that Jesus said, in that a man's evil or good is all in his intention, not in that which is external to him. Not foods. Not physically touching a woman—intending or desiring to, which is equivalent here in God's eyes, and in our conscience, to which the New Covenant gives much focus. 4
3 Heb 11:19
4 Mt 5:28
The playing out of the good or bad intentions of the heart (by which is meant works of whatever kind) are, so to speak, 'additional evils,' in that they create more offense to God than He would have to otherwise bear from your being prevented from having worked out your evil intention: tempting others to the same sin, showing bad example, degrading your body and the temple of the Holy Ghost, etc.
One is reminded of a related Scripture passage:
1 Samuel 16:7 (cf. Lk 16:15)
And the Lord said to Samuel: Look not on his countenance, nor on the
height of his stature: because I have rejected him, nor do I judge
according to the look of man: for man seeth those things that appear,
but the Lord beholdeth the heart.
Why Foods Cannot Be Intrinsically Evil
Foods cannot be intrinsically evil any more than Satan was intrinsically evil before he misused his will. Any more than a gun is intrinsically evil. Any more than a sword, etc. It takes an agent (someone with a will) to will evil from the heart, who, if he is not hindered, he will commit an 'additional evil' of actually carrying out that evil intention for which, in God's eyes, he has already murdered.
This is why Our Lord makes gives us the following shocking doctrine, as recounted by St. John:
1 John 3:15 (from Our Lord's teaching in Mt 5:21-22):
Whosoever hateth his brother is a murderer. And you know that no
murderer hath eternal life abiding in himself.
This is because God knows that on some level, we sometimes wish our brother were dead, when we hate him. Not in an explicit thought, but as a vague desire in the heat of the moment, as it were. Jesus wants us to avoid have such a heinous, even instantaneous, desire. Which in His eyes, because He sees it for what it is, it is murder.
Similarly, it cannot be sin in and of itself to eat a lobster. But what can be even gravely sinful is directly disobeying God who said (for reasons other than that it intrinsically offends him) you are not to eat this thing.
God made it sinful to eat lobster not because He doesn't like lobsters, nor did God want the life of Isaac. ('Killing Isaacs' isn't a way to be justified, for example). He wants obedience in those things He asks, no matter what they are: even seemingly arbitrary things. And in obedience to God's laws will bring you into right relationship with Him: being justified.