Is it a good or bad sign that meditation is sometimes a frightening and nervous activity?

If you have crossed Arising and Passing away (mistakenly taken as kundalini, or enlightenment experience) Yes, it is expected you go through it at some stage.

refer to:

https://www.accesstoinsight.org/lib/authors/mahasi/progress.html#ch6.6

https://www.dharmaoverground.org/dharma-wiki/-/wiki/Main/MCTB+6.+Fear

Meditation is an activity of 'mental solitude', which will bring up fear & nervousness. Fear is an instinctual emotion that probably every practitioner that "crosses over" must pass through when the instinctual 'self' or 'ego' is diminished.

For people with a history or disposition of mental instability, they probably need to be careful here and should ideally meditate with a group.

Marijn and who ever might be interested,

assuming that the question was raised in seek for release:

First:

❋ Namo tassa bhagavato arahato sammā-sambuddhassa ❋

Restlessness and Remorse (nervous or scared) is one of the five hindrences and ne needs - aside of it being perceived, become aware of it - by coming aware of it, knowing it as hindrence, be abounded.

The means and ways for that are found in a compiled work here, based on teachings by the Buddha, found in the canon: The Five Mental Hindrances and Their Conquest

The two mental faculties (indriya), right concentration (samadhi) and right mindfullness (sati) are the means to abound it.

The cause of concentration is rature, the cause of rapture is freedom from remorse, or conviction.

How is the freedom from remorse developed: by acts of generosity, by keeping the precepts without spot.

And how is conviction developed? By reminding the Buddhas quality, the Dhammas quality, the Sanghas quality, by reminding ones generosity, by reminding ones virtue, by reminding, calling in mind, ones virtues equal those of the devas. (The six reflections)

So by taking/asking for refuge, by taking/asking for the precepts, by devotion and acts of it as preperation, the bas*m*nt is developed.

See for this required proper prepairation: Respect, Confidence and Patient and probably Devotion, basicly requirement because without respect and gratitude (e.g. right view) no right object of joy, no joy no right concentration, no right concentration no release. So is it that right view is the forrunner for right release and by wrong view wrong releas comes into be.

And what is right view?

There is the case where a certain person is not covetous. He does not covet the belongings of others, thinking, 'O, that what belongs to others would be mine!' He bears no ill will and is not corrupt in the resolves of his heart. [He thinks,] 'May these beings be free from animosity, free from oppression, free from trouble, and may they look after themselves with ease!' He has right view and is not warped in the way he sees things: 'There is what is given, what is offered, what is sacrificed (convition in result of action, benefit of giving). There are fruits & results of good & bad actions (convition in fruits of kamma, benefit of dana, sila, bhavana). There is this world & the next world (convition in rebirth). There is mother & father (respect and gratitude torward giver, goodness). There are spontaneously reborn beings (convition in Devas, deities and their virtues); there are brahmans & contemplatives who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.(devotion torward the juwels, following them, relay on them)' This is how one is made pure in three ways by mental action."

— AN 10.176

Q: But is this (scared or nervous) a good or bad sign?

Is just a Dhamma. Seeing Dhammas arise and decay. Knowing there qualities, holding on, increase developing benefical and giving no further attention, abound unbenefical, is the work not only to reach proper concentration, Jhana, but also to obsereve them (Dhammas) there in there arising and decay more refined and like they actually are.

It's a good sign, being aware of it, and for its abounding, it was a good sign, a blessing, to ask abot it:

Six things are conducive to the abandonment of restlessness and remorse:

1. Knowledge of the Buddhist scriptures (Doctrine and Discipline);

2. Asking questions about them;

3. Familiarity with the Vinaya (the Code of Monastic Discipline, and for lay followers, with the principles of moral conduct);

4. Association with those mature in age and experience, who possess dignity, restraint and calm;

5. Noble friendship;

6. Suitable conversation.

So now it's up to your right effort to make the best out of your previous and here rippened good sign.

Mudita

[Note: This is a gift of Dhamma, not meant for commercial purpose and other low wordily gains by means of trade and exchange]