If you frequently make 'thin value' bets on the river when you only have a decent hand, shouldn't observant opponents pick up on this and try to trap you more with their very good hands?

Better opponents have a balanced range, so it's harder to bet for thin value when you know villain will simply call down with some decent hands.

If you constantly bet-fold, an obvious adjustment is to raise with semi-bluffs and watch you fold everything except the nuts. So your counter-adjustment is not to go for thin value as much, start checking some good hands, go for two streets instead of three etc. etc. Which effectively means you are balancing your range.

Basically depends on your opponents. Against loose-passive opponents that play low stakes live games, betting for fat value and bet-folding one pair hands are extremely effective. Against anyone who can play poker, not so much.

In a word, yes. If you bet weakish hands too often, a villain can exploit you, both by checking to induce with better, and by bluff-raising.
Most micro villains aren't going to get a large enough sample size to realize your betting frequencies are "too high" though, let alone be able to work out the best way to exploit your 'leak'.
Players that are already balanced, however, will make money from you by default, since the good/balanced player will already have a solid checking range and you'll fall into his 'trap' without him consciously setting it.

more the case when you bluff

Most opponents are not observant, so if it seems that thin value betting is a good idea, keep doing it until they counter-exploit you with traps. Then, be prepared to switch gears.

Just because a strategy is open to being countered, that doesn't mean you should avoid it.

Those really don't go together. Thin value (to me) comes from trying to get value when you have a marginal hand (2nd/3rd pair, no kicker) or you are betting even though the board is really telling you not to bet (4-flush/straight, pair/double).

As long as you are betting for 'real' value (and in the same manner) as often as you go for thin value even the best opponents will think twice about playing back at you without a better hand against your range .. and the board's range. The amount of chips you miss out on when they raise with a lessor hand and you fold should be far less than the extra you pick up when they do call with the weakest of their range. GL

It takes a better hand to bet for thin value against a tight opponent than against a loose opponent. It depends on your opponent's range. A value bet is one that is profitable against your opponent's range. Thin value is a value bet that is only slightly profitable against your opponent's range.

It kind of depends what OP meant by "decent". In some contexts (or for some people) "decent" means more or less the same as "marginal".

In today's games, TPTK (but more commonly TPGK) is a "decent hand", but it's often a marginal value-bet on the river*. In most spots, if you bet second or third pair it would be as a bluff, not a (crazily thin) value bet.

* On many runouts TPTK will be something like the 20th nuts by the river, so it's far from being a slam-dunk value bet, because it can be very hard to get called by worse more than 50% of the time.